* fix(discord): hydrate channel context when replying to a message
Replying to a message in a free-response (non-mention, threads-off)
channel previously received only the 500-char "[Replying to: ...]"
snippet — the history-backfill gate fired only for mention-gated
channels and threads, so a reply got no surrounding channel context.
Replies now route through the same _fetch_channel_context hydration
that threads use. When the user replied to a specific (often older)
message, a reply-anchored window is scanned ending at that message so
the agent sees the exchange around what was pointed at, even when the
target sits before the self-message partition. The two windows are
merged chronologically and de-duplicated by message id.
Also hardens the recent-window scan to skip non-conversational status
bumps before the self-message partition check, and makes author-name
resolution defensive against partial/deleted authors.
* fix(discord): duck-type reply-target resolution instead of isinstance(discord.Message)
The e2e suite stubs the discord module, so discord.Message is a MagicMock
and isinstance(_resolved, discord.Message) raises 'isinstance() arg 2 must
be a type'. Any object with an int .id works as a scan anchor, so resolve
the reply target by duck-typing on .id and fall back to a _Snowflake from
the reference message_id.
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