fix: reject mismatched AI provider fallbacks (F5)

[Pigbibi]

Summary

• Report review unavailable without ai_gateway_client rather than submit independent-review tasks to Codex execution.
• Reject non-Codex providers and non-execute tasks before authentication or HTTP in local fallback.
• Preserve legitimate Codex execute/verify in review_only mode, using endpoint identity instead of arbitrary caller labels.

Validation

• Offline tests-first: 13 failures on original source, then 47 passed and 95 subtests passed across provider, reviewer, Codex integration, CLI and candidate notification regressions.
• Targeted ruff and git diff --check passed. Normal SDK analyze provider identity and local Codex execute/verify remain covered.
• No provider calls, paid AI, credentials, deployment, public schema or dependency changes.

Adoption boundary

Only the provider module and its existing tests change. QPK_PIN remains unchanged. External strategy pins and their independent drift review workflow are not upgraded by this change; source fix is not production adoption evidence.