fix(opencode): expose config parse errors over http

[YOMXXX]

Issue for this PR

Closes #29200

Type of change

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

What does this PR do?

Config JSON/JSONC parse and schema validation errors can be thrown as defects during startup request fanout. The generic HTTP error middleware was converting those defects into an opaque 500 response, so clients only saw Unexpected server error.

This keeps the generic 500 behavior for unknown and unrelated named defects, but returns structured ConfigJsonError and ConfigInvalidError responses with HTTP 400 so the CLI/TUI can render the actual config path and validation issue.

How did you verify your code works?

• bun test test/server/httpapi-error-middleware.test.ts from packages/opencode
• bun typecheck from packages/opencode
• bun turbo typecheck from repo root
• .husky/pre-push from repo root

Screenshots / recordings

Not applicable.

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:

Potential Duplicate Found

PR #29208 - fix(config): catch parse errors gracefully during startup
#29208

Why it might be related: This PR also addresses config parse errors during startup, which is directly related to PR #29238's focus on exposing config parse errors (both JSON/JSONC parsing and schema validation) over HTTP with proper error responses.

[YOMXXX]

Thanks for the duplicate check. This is related to #29208, but it takes a different approach.

#29208 changes config loading to swallow parse/schema failures and fall back to defaults. This PR keeps config parse/schema failures intact and only maps ConfigJsonError / ConfigInvalidError defects at the HTTP boundary to structured 400 responses, so startup request aggregation can show the actual config path and validation issue instead of a generic 500.

The regression coverage is in test/server/httpapi-error-middleware.test.ts, and unrelated named defects still return the safe generic 500 body.