fix(proxy): unify eventless anchor poison settlement

[hongzexin]

Summary

Unify eventless HTTP bridge failure settlement so the retry circuit itself owns poisoned-anchor clearing. This closes the reader/terminal mixed-order window where two eventless failures opened cooldown but the old durable anchor could still be re-injected.

Type of change

• fix: — bug fix (no behavior change beyond the bug)

OpenSpec

• This PR includes / updates an OpenSpec change
• This PR touches a codex-faithful path and preserves upstream-equivalent behavior

Change directory: openspec/changes/unify-eventless-anchor-poison-settlement/

Changes

• Reader, terminal, grouped-terminal, and stale-retirement paths share the attempt-scoped retry circuit.
• Circuit-open poison settlement uses fenced durable anchor clear with idempotent duplicate handling.
• Clear failure remains fail-closed across cooldown expiry and process restart; full replay can run unanchored while delta-only requests return 503.
• Successful completed responses reset poison state.
• Added mixed-order, duplicate-callback, clear-failure, restart-marker, and cooldown regression coverage.

Test plan

make lint
make typecheck
pnpm dlx @fission-ai/openspec@1.11.0 validate --specs
uv run pytest -q tests/unit/test_proxy_http_bridge.py
uv run pytest -q tests/integration/test_http_responses_bridge.py

Results: lint, typecheck, OpenSpec 57/57, unit 686 passed, integration 151 passed.

Source sidechat: 01a04ac6-27a1-74b3-88d3-f1ede3901184
Recovered from: d1f4fa4