fix(agent): align preflight token-progress floor to 5% (#23767, #39548)

Follow-up to the salvaged preflight token-progress fix: require a material
(>5%) token reduction to count as progress, matching the overflow-handler
retry path (conversation_loop.py, #39550), so a sub-5% wobble can't keep the
3-pass preflight loop spinning. Adds boundary + zero-token regression tests.

agent/turn_context.py

@@ -47,8 +47,14 @@ def _compression_made_progress(
     context window.  See issue #39548 for an observed case: 220 → 220
     messages, ~288k → ~183k tokens on a 1M-context model still triggered
     auto-reset.
+
+    The token reduction must be *material* (>5%) to count as progress — the
+    same floor the overflow-handler retry path uses (conversation_loop.py,
+    #39550) — so a sub-5% wobble doesn't keep the multi-pass loop spinning.
     """
-    return new_len < orig_len or new_tokens < orig_tokens
+    if new_len < orig_len:
+        return True
+    return orig_tokens > 0 and new_tokens < orig_tokens * 0.95
 
 
 @dataclass

tests/agent/test_compression_progress.py

@@ -7,8 +7,9 @@ estimated request token count without removing any rows — and surfaces a
 spurious ``Context length exceeded`` failure followed by an auto-reset of
 an otherwise healthy session.
 
-These tests pin the contract of ``_compression_made_progress``: either a
-row-count reduction OR a token-count reduction counts as progress.
+These tests pin the contract of ``_compression_made_progress``: a
+row-count reduction OR a *material* (>5%) token-count reduction counts as
+progress.
 """
 
 from __future__ import annotations
@@ -64,3 +65,22 @@ class TestCompressionMadeProgress:
         assert _compression_made_progress(
             orig_len=10, new_len=5, orig_tokens=1000, new_tokens=1100
         ) is True
+
+    def test_sub_5pct_token_drop_is_not_progress(self):
+        """A token reduction below the 5% material floor does NOT count as
+        progress — matching the overflow-handler retry path (#39550) so a
+        marginal wobble can't keep the multi-pass loop spinning."""
+        # 1000 -> 970 is a 3% drop, below the 5% floor.
+        assert _compression_made_progress(
+            orig_len=10, new_len=10, orig_tokens=1000, new_tokens=970
+        ) is False
+        # 1000 -> 940 is a 6% drop, above the floor.
+        assert _compression_made_progress(
+            orig_len=10, new_len=10, orig_tokens=1000, new_tokens=940
+        ) is True
+
+    def test_zero_orig_tokens_is_not_progress(self):
+        """Degenerate estimate (0 tokens) must not be read as a token win."""
+        assert _compression_made_progress(
+            orig_len=10, new_len=10, orig_tokens=0, new_tokens=0
+        ) is False