fix: preserve OpenAI OAuth prompt cache affinity

[2314860760]

Description

OpenAI OAuth requests use stateless Responses API calls and already carry the Chatbox session ID through the chat orchestration layer. However, the session ID was not propagated to OpenAI's prompt_cache_key option, so consecutive requests could lose prompt-cache affinity.

This change:

• Uses the existing session ID as promptCacheKey for stateless OAuth Responses requests.
• Preserves store: false for OAuth requests.
• Avoids changing behavior for regular API-key Responses providers.
• Adds regression tests for session and missing-session cases.

Testing

• Targeted Vitest tests passed (4/4)
• Biome check passed
• TypeScript check passed
• Electron production build passed
• git diff --check passed

The full local suite also exposed unrelated environment-specific failures in Electron dependency installation and one Windows path-separator assertion; no failures were caused by the changed files.

Additional Notes

This uses the official OpenAI Responses API prompt_cache_key provider option and does not modify OAuth endpoints, token handling, or authentication behavior.

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• I have read and agree with the above statement.

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