feat(api): add system_instruction and conversation continuity

[seanbrar]

Summary

Adds a global system_instruction configuration option and implements conversation continuity (history and continue_from) specifically for the OpenAI provider in v1.1.

Related issue

None

Test plan

I have verified these changes locally using the project's verification suite.

• Architectural Guarantee / Boundary Coverage: Added new E2E API logic in tests/test_api.py testing the system instructions and roundtripping of the conversation state.
• Trivial Delegation: Ensured OpenAI parameters are accurately mapped and routed to the OpenAI Responses API (tests/test_providers.py).
• Integration: Ran make check locally, confirming 100% pass rate for Ruff format/linting, MyPy type checks, and 71 Pytest test cases.

Notes

• delivery_mode="deferred" remains intentionally reserved.
• Conversation continuity is currently OpenAI-only, as Gemini is not yet supported for this feature in Pollux. The provider capability matrix has been updated to reflect this explicitly.

[codecov]

Codecov Report

❌ Patch coverage is 95.65217% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/pollux/providers/openai.py	88.88%	2 Missing ⚠️

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