fix: allow backend Codex HTTP fallback with session headers

[kgskr]

Summary

• allow stateless backend Codex HTTP fallback even when downstream session_id / x-codex-* headers are present
• keep payload-level continuity (conversation, previous_response_id) fail-closed for Platform fallback
• document the backend Codex HTTP fallback scope and bump the release version to 1.12.5

Details

• update backend Codex continuity detection so transport/session headers do not suppress eligible Platform fallback
• add unit and integration coverage for backend Codex HTTP fallback with session headers
• update OpenSpec and README to match the implemented routing and fallback policy
• bump package version metadata in pyproject.toml, app/__init__.py, and release manifest

Validation

• ruff check app/modules/proxy/api.py tests/unit/test_public_route_selection.py tests/integration/test_openai_platform_proxy.py
• pytest tests/unit/test_public_route_selection.py -q
• pytest tests/integration/test_openai_platform_proxy.py -k 'turn_state_header_uses_platform_fallback or responses_uses_platform_when_force_fallback_is_enabled or responses_rejects_continuity_when_only_platform or responses_uses_platform_when_backend_http_usage_is_drained' -q
• openspec validate --specs

[kgskr]

@codex

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. You're on a roll.

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