Keep A2A canceled sticky against message/send persist

[cursor]

Problem

tasks/cancel persists canceled while an in-flight message/send is still unwinding. The stream path already refuses to overwrite that row (get_a2a_task then skip the terminal persist). The unary send path always wrote build_terminal_task's completed/failed through update_a2a_task, so a successful (or failed) send could revive a canceled task. tasks/get then disagreed with the cancel RPC, and the send client received a completed Task.

Exception / tenant-disabled paths on send had the same hole: they wrote failed without checking for a racing cancel.

Fix

update_a2a_task keeps canceled sticky: a non-canceled state is a no-op when the row is already canceled. Re-writing canceled (the cancel handler itself) still lands so updated_at / error stay current. Unary message/send treats that CAS miss as request cancelled instead of returning a completed Task.

Tests

unit_request_log — canceled is not overwritten by completed/failed; re-writing canceled still succeeds; working → completed still works when cancel never landed.

Independently mergeable against main. Code hunks are disjoint from #322/#330 (list_conversations / EntryFilter) and from #321–#329. CHANGELOG [Unreleased] will need a rebase after whichever sibling merges first.

  

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Note

Medium Risk
Changes durable A2A task state and unary RPC outcomes on cancel/send races; behavior is narrower (cancel wins) but affects protocol clients that relied on a completed send response after cancel.

Overview
Fixes a race where tasks/cancel could persist canceled while unary message/send was still finishing, then update_a2a_task wrote completed / failed and the RPC returned a terminal Task that tasks/get contradicted. The stream path already avoided that overwrite; this change aligns unary send.

TenantStore::update_a2a_task now treats canceled as sticky in SQL: updates with a non-canceled state are a no-op when the row is already canceled; re-writing canceled (e.g. from tasks/cancel) still succeeds so updated_at / error text stay current. Unary message/send checks that CAS: if persist fails and the row is still canceled, it responds with request cancelled instead of a completed Task.

Adds a unit_request_log test for sticky cancel plus normal working → completed when cancel never landed, and documents the behavior in CHANGELOG and tenant_store.h.

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