fix: improve Gemini caching through OpenRouter

[AbdoKnbGit]

Issue for this PR

Closes#36069

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Gemini requests through OpenRouter were not using the explicit cache breakpoints already supported by OpenCode.

This enables the existing cache-control behavior for OpenRouter Gemini models and includes the OpenCode session ID in OpenRouter session_id field to improve sticky routing and cache reuse.

How did you verify your code works?

Ran bun test test/provider/transform.test.ts from packages/opencode with 295 tests passing.

I also inspected the generated OpenRouter request and confirmed it includes the expected cache_control metadata and top-level session_id.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Related PR Found:

PR #35982: fix(provider): improve prompt caching
#35982

This PR is closely related as it also addresses prompt caching improvements at the provider level. While this current PR (#36070) specifically focuses on enabling Gemini caching through OpenRouter with session ID inclusion, PR #35982 appears to be a broader provider-level caching improvement that may overlap in scope or provide relevant context.

[github-actions]

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