fix(provider): opencode-go context limits and 400 error surfacing (#50446) - #52731

Issue for this PR

Closes #50446

Type of change

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

What does this PR do?

• Caps opencode-go catalog limits to 148k context and 128k input in packages/core/src/plugin/models-dev.ts with a clamp in packages/core/src/session/runner/model.ts so automatic compaction fires before hitting the gateway's input ceiling.
• Expands context overflow classification in packages/llm/src/provider-error.ts and packages/llm/src/protocols/anthropic-messages.ts to recognize gateway input length rejections.
• Surfaces and preserves provider HTTP 400 error response bodies in packages/llm/src/route/executor.ts and packages/opencode/src/provider/error.ts instead of swallowing them into a bare status message.
• Adds regression unit tests covering opencode-go gateway limits, error body preservation, and context overflow classification.

How did you verify your code works?

• Tested with bun test test/opencode-go-gateway.test.ts in packages/llm (3 pass, 0 fail).
• Tested with bun test test/provider/error.test.ts in packages/opencode (4 pass, 0 fail).
• Tested with bun test test/plugin/models-dev.test.ts in packages/core (3 pass, 0 fail).
• All tests pass cleanly.

Screenshots / recordings

N/A

Checklist

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• I have added tests to cover my changes.
• All new and existing tests passed.