fix(cache): align Codex session_id/x-client-request-id headers with body key

[JoaoMarcos44]

Summary

Fixes NousResearch#79013. Split out of NousResearch#78959's continuation for standalone review.

ResponsesApiTransport.build_kwargs() sent both Codex-backend headers (session_id, x-client-request-id) as the same normalized scope string (e.g. cron_job42), while the body's prompt_cache_key is a content hash (pck_<hash>). Two problems:

1. session_id header lost the raw physical id, contradicting the Align Codex cache affinity header NousResearch/hermes-agent#57012 contract (session_id = transcript identity, x-client-request-id = cache-routing key).
2. x-client-request-id never actually matched the key the body was cached under.

Fix

session_id header = raw physical session_id, untouched. x-client-request-id = the same effective (already-bounded) prompt_cache_key sent in the body. Also fixed the fallback cache_key (used when instructions/tools are empty) to fall back to the normalized scope instead of the raw session_id -- same class of bug.

Test plan

• test_codex_cache_scope_boundary — session_id header stays raw; x-client-request-id mirrors body key
• test_codex_cache_scope_headers_normalize_cron_session_id — x-client-request-id stable across cron re-fires of the same job; session_id stays per-fire physical
• Full test_codex_transport.py suite green (68 tests)

%%{init: {'theme': 'dark', 'themeVariables': { 'primaryColor': '#00f0ff', 'mainBkg': '#0a0a16', 'primaryTextColor': '#ffffff', 'primaryBorderColor': '#ff007f', 'lineColor': '#00f0ff'}}}%%
graph TD
    A[Codex Request] --> B[body.prompt_cache_key<br/>pck_content_hash]
    A --> C[header.session_id<br/>Before: normalized scope]
    A --> D[header.x-client-request-id<br/>Before: normalized scope]

    C -->|Mismatch| E[Broken #57012 Contract]
    D -->|Mismatch| F[Diverges From Body Key]

    A --> G[header.session_id<br/>After: raw physical id]
    A --> H[header.x-client-request-id<br/>After: mirrors body key]
    G --> I[Transcript Identity Preserved]
    H --> J[Header/Body Agreement]

    style E fill:#3a0030,stroke:#ff007f,color:#fff
    style F fill:#3a0030,stroke:#ff007f,color:#fff
    style I fill:#003a3a,stroke:#00f0ff,color:#fff
    style J fill:#003a3a,stroke:#00f0ff,color:#fff

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[JoaoMarcos44]

Folded back into the upstream PR (NousResearch#78959) so maintainers review everything in one visible upstream PR instead of fork-only ones. Closing.