fix(opencode): gate provider 400 rate limit retries

[maximkr]

Issue for this PR

Fixes #40241

Type of change

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

What does this PR do?

When an organization uses a one-minute LLM limit window, some providers report the exhausted window as a non-standard HTTP 400 response with a JSON detail containing rate limit exceeded. OpenCode previously treated that response as terminal and stopped session processing.

This PR adds the provider-local opt-in provider.<id>.options.retry400RateLimit. When enabled, the existing retry flow handles only matching HTTP 400 rate-limit responses. Generic HTTP 400 responses remain non-retryable, and the option is not passed through as an SDK request option.

The implementation threads the setting through SessionProcessor and MessageV2.fromError into ProviderError.parseAPICallError, with unit and processor integration tests plus design documentation.

How did you verify your code works?

• bun test test/session/retry.test.ts test/session/processor-effect.test.ts --timeout 30000 from packages/opencode — 52 passed, 0 failed.
• bun typecheck from packages/opencode — passed.
• git diff --check — passed.
• The repository-wide pre-push typecheck is currently blocked by an unrelated existing TS1128 error in packages/enterprise/src/custom-elements.d.ts.

Screenshots / recordings

Not applicable; this is a server/session error-handling change.

Checklist

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

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