fix(agent): route content-filter stream stalls to fallback chain (#32421)

When a provider's output-layer safety filter (MiniMax "output new_sensitive
(1027)", Azure content_filter, etc.) kills a streaming response after deltas
were already sent, interruptible_streaming_api_call swallows the raw error
into a finish_reason=length partial-stream stub. The conversation loop then
burned 3 continuation retries against the SAME primary — re-hitting the
content-deterministic filter every time — and gave up with "Response remained
truncated after 3 continuation attempts", never consulting fallback_providers.

Builds on @595650661's classifier change (cherry-picked) so error_classifier
recognizes the filter; then:
- chat_completion_helpers: run the swallowed error through error_classifier at
  the stub-creation point and stamp _content_filter_terminated on the stub
  (single source of truth — no parallel pattern list).
- conversation_loop: read the tag and activate the fallback chain BEFORE
  burning any continuation retries; roll partial content back to the last
  clean turn and re-issue against the new provider (restart_with_rebuilt_messages).
  Plain network stalls are unaffected (only content_policy_blocked is tagged).

Credits #32479 (@sweetcornna) and #33845 (@Tranquil-Flow) which fixed the
same issue via the stub-tag and loop-escalation approaches respectively.

Live E2E confirmed: before, _try_activate_fallback called 0x; after, fallback
fires on the first stub and the fallback provider completes the turn.

scripts/release.py

@@ -64,6 +64,7 @@ AUTHOR_MAP = {
     "8180647+herbalizer404@users.noreply.github.com": "herbalizer404",  # PR #49076 + #51835 salvage (auxiliary compression fallback: 403/session-usage payment errors + honor fallback chain when aux provider auth unavailable)
     "pyxl-dev@users.noreply.github.com": "pyxl-dev",  # PR #52230 salvage (include rate-limit in auxiliary capacity-error fallback gate; #52228)
     "yashiel@skyner.co.za": "yashiels",  # PR #53284 salvage (discord markdown table-to-bullet conversion; #21168)
+    "15205536+595650661@users.noreply.github.com": "595650661",  # PR #37851 salvage (classify MiniMax new_sensitive content filter → content_policy_blocked; #32421)
     "benbenwyb@gmail.com": "benbenlijie",  # PR #47205 salvage (named custom-provider extra_body + Z.AI Coding overload adaptive backoff; #50663)
     "dana@added-value.co.il": "Danamove",  # PR #46726 salvage (kill venv-resident pythonw gateway before recreating venv on Windows; #47036/#47557/#47910)
     "145739220+wgu9@users.noreply.github.com": "wgu9",  # PR #51468 salvage (WSL/no-systemd orphan gateway tracking, #51325)