fix: negotiate unsupported sampling controls across transports

[seonghobae]

Summary

• retry once without temperature when the selected provider rejects that optional control with HTTP 400/422
• apply the same capability negotiation to Chat Completions, Responses, and raw/proxy transport
• keep model selection, fallback tables, and reasoning policy upstream-owned; no model-name heuristic

Evidence

• uv run pytest tests/test_provider_protocol.py tests/test_provider_integration.py -q: 16 passed
• full upstream suite on this feature head: 459 passed
• git diff --check: passed

Decision record

• docs/planning/adrs/0012-optional-sampling-capability-negotiation.md

This PR is intentionally scoped as a stack on feat/paper-grounded-auto-orchestration-20260820; it does not modify or force-push draft PR #765. Hosted Checks are not materialized for this non-default base, so this evidence is not a protected-main merge claim.

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[seonghobae]

Reviewed and fixed the current stack head. The shared is_provider_capability_error remains generic for structured-output negotiation; temperature retries now require a 400/422 response whose reason/body explicitly says temperature is unsupported. This prevents retrying invalid temperature values while preserving existing JSON-schema/json-object fallback. Exact HEAD 8977384; focused provider/protocol/passthrough tests: 40 passed; full suite: 460 passed; compileall and git diff --check passed. Please review this exact SHA only.

[seonghobae]

Current-head review: head 8977384 is based on closed branch feat/paper-grounded-auto-orchestration-20260820 (PR #766), so GitHub reports no required Checks. The current successor #765 does not contain the old provider_protocol field; retargeting without a rebase would expand this into a 72-file diff. Please rebase/cherry-pick the two capability-negotiation commits onto the current #765/main integration line, preserve the 16 focused tests, and make the exception-chain walk honor suppress_context so raise-from-none cannot trigger an unrelated temperature retry. Re-run hosted Checks and obtain independent approval before merge.

[seonghobae]

Successor PR #779 is open on the exact current #765 integration head. It reapplies only the temperature capability negotiation at the current common chat/raw transport boundary, with 1468 full-suite passes. #774 remains intentionally unretargeted and unmerged because its closed-base history has no hosted required Checks; no force-push or protection bypass was used.

[seonghobae]

Superseded by #779, which rebases the same optional-temperature capability negotiation onto the current #765 integration line and restores hosted exact-head Checks. Closing this stale-base duplicate unmerged; no evidence from this head transfers to #779.