refactor(gateway): share error-text blob between not_found classifiers

Follow-up to the #55780 dead-target not_found blast-radius fix (merged in
#56225). classify_send_error and is_chat_level_not_found each built their own
lowercased error blob, but divergently: classify_send_error appended the
exception CLASS NAME while is_chat_level_not_found did not. A caller passing
exc= to both could get inconsistent answers on the same failure.

- Extract _error_blob(exc, error_text) as the single source of truth both
  classifiers use (str(exc) when non-empty + class name; no stray leading
  space).
- Align is_chat_level_not_found's signature to (exc, error_text), matching
  classify_send_error, removing the swapped-positional footgun; update the
  sole caller and the three tests to keyword form.
- Add a regression guard asserting _error_blob keeps the class name.

Surfaced by the hermes-pr-review Phase 2c structured review of #56225.

gateway/delivery.py

@@ -133,7 +133,7 @@ def _classify_dead_from_error_text(error_text: Optional[str]) -> Optional[str]:
     # Only a whole-chat not_found means the target is dead — a deleted forum topic
     # or an edited-away message must not mark the entire chat (and all of its future
     # deliveries) dead.  See gateway.dead_targets' documented scope.
-    if kind == "not_found" and not is_chat_level_not_found(error_text):
+    if kind == "not_found" and not is_chat_level_not_found(error_text=error_text):
         return None
     return kind
 

gateway/platforms/base.py

@@ -1931,6 +1931,26 @@ _SUBCHAT_NOT_FOUND_SUBSTRINGS = (
 )
 
 
+def _error_blob(exc: Optional[BaseException] = None, error_text: str = "") -> str:
+    """Build the lowercased text blob both send-error classifiers match against.
+
+    Single source of truth so ``classify_send_error`` and
+    ``is_chat_level_not_found`` can never drift (e.g. one including the
+    exception class name and the other not) and silently disagree on the same
+    failure.  Includes ``str(exc)`` (when non-empty) and the exception's class
+    name, plus any explicit ``error_text``.
+    """
+    parts = []
+    if error_text:
+        parts.append(error_text)
+    if exc is not None:
+        exc_str = str(exc)
+        if exc_str:
+            parts.append(exc_str)
+        parts.append(exc.__class__.__name__)
+    return " ".join(parts).lower()
+
+
 def classify_send_error(exc: Optional[BaseException], error_text: str = "") -> str:
     """Map a send exception / error string to a :data:`SEND_ERROR_KINDS` value.
 
@@ -1939,13 +1959,7 @@ def classify_send_error(exc: Optional[BaseException], error_text: str = "") -> s
     use.  Conservative — anything unrecognized returns ``"unknown"`` so callers
     never mistake an unclassified failure for a benign one.
     """
-    parts = []
-    if error_text:
-        parts.append(error_text)
-    if exc is not None:
-        parts.append(str(exc))
-        parts.append(exc.__class__.__name__)
-    blob = " ".join(parts).lower()
+    blob = _error_blob(exc, error_text)
     if not blob.strip():
         return "unknown"
     if "message_too_long" in blob or "too long" in blob or "message is too long" in blob:
@@ -1988,7 +2002,7 @@ def classify_send_error(exc: Optional[BaseException], error_text: str = "") -> s
     return "unknown"
 
 
-def is_chat_level_not_found(error_text: str = "", exc: Optional[BaseException] = None) -> bool:
+def is_chat_level_not_found(exc: Optional[BaseException] = None, error_text: str = "") -> bool:
     """Whether a ``not_found`` failure means the *whole chat* is gone.
 
     :func:`classify_send_error` collapses chat-level and thread/topic/message-level
@@ -1997,13 +2011,12 @@ def is_chat_level_not_found(error_text: str = "", exc: Optional[BaseException] =
     forum topic or an edited-away message leaves the parent chat reachable.  When
     both a chat-level and a sub-chat marker are present, the sub-chat reading wins
     (conservative: never kill a chat that may still be reachable).
+
+    Argument order mirrors :func:`classify_send_error` (``exc`` first) and both
+    share :func:`_error_blob`, so the two classifiers cannot disagree on the same
+    failure.
     """
-    parts = []
-    if error_text:
-        parts.append(error_text)
-    if exc is not None:
-        parts.append(str(exc))
-    blob = " ".join(parts).lower()
+    blob = _error_blob(exc, error_text)
     if any(s in blob for s in _SUBCHAT_NOT_FOUND_SUBSTRINGS):
         return False
     return any(s in blob for s in _CHAT_LEVEL_NOT_FOUND_SUBSTRINGS)

tests/gateway/test_dead_targets.py

@@ -225,19 +225,19 @@ class TestNotFoundBlastRadius:
     def test_is_chat_level_not_found_chat_level(self):
         from gateway.platforms.base import is_chat_level_not_found
 
-        assert is_chat_level_not_found("Bad Request: chat not found") is True
+        assert is_chat_level_not_found(error_text="Bad Request: chat not found") is True
 
     @pytest.mark.parametrize("message", _SUBCHAT_NOT_FOUND_MESSAGES)
     def test_is_chat_level_not_found_subchat(self, message):
         from gateway.platforms.base import is_chat_level_not_found
 
-        assert is_chat_level_not_found(message) is False
+        assert is_chat_level_not_found(error_text=message) is False
 
     def test_subchat_marker_wins_when_both_present(self):
         from gateway.platforms.base import is_chat_level_not_found
 
         # Conservative: if a sub-chat marker is present, never kill the whole chat.
-        assert is_chat_level_not_found("chat not found; message thread not found") is False
+        assert is_chat_level_not_found(error_text="chat not found; message thread not found") is False
 
     def test_classify_dead_from_error_text_gates_not_found(self):
         from gateway.delivery import _classify_dead_from_error_text
@@ -246,3 +246,19 @@ class TestNotFoundBlastRadius:
         assert _classify_dead_from_error_text("Bad Request: chat not found") == "not_found"
         assert _classify_dead_from_error_text("Bad Request: message thread not found") is None
         assert _classify_dead_from_error_text("httpx.ReadTimeout: connection timed out") is None
+
+    def test_error_blob_is_shared_source_of_truth(self):
+        # Regression guard: classify_send_error and is_chat_level_not_found must
+        # both derive their match text from the SAME _error_blob helper (which
+        # includes the exception CLASS NAME), so they can never drift. Before
+        # this consolidation is_chat_level_not_found built its own blob from
+        # str(exc) only, omitting the class name classify_send_error included.
+        from gateway.platforms import base
+
+        class TopicDeleted(Exception):
+            pass
+
+        # Empty message: the only signal is the class name — _error_blob keeps it,
+        # with no stray leading space from an empty str(exc).
+        assert base._error_blob(TopicDeleted()) == "topicdeleted"
+        assert base._error_blob(TopicDeleted("boom")) == "boom topicdeleted"