Normalize OpenCode Go null tool calls

[TheGreatAxios]

Summary

• OpenCode Go treats nullable tool-call deltas as absent
• Other OpenAI-compatible sources retain strict response validation
• The provider quirk remains explicit and upstreamable

Verification

• bun run check passes
• Focused adapter, source, and vendored parser tests pass

Refs CL-7367

[linear-code]

CL-7367

[TheGreatAxios]

Reviewed the full origin/main...HEAD diff. No blocking or should-fix findings.

The OpenCode Go source opts into null tool-call normalization while default and malformed non-null values remain strict. Focused adapter, source, and vendored parser tests pass.

[TheGreatAxios]

Superseded by #753. Current main routes OpenCode Go chat-completions through a dedicated adapter that normalizes null role and tool_calls fields before the strict OpenAI parser, while malformed non-null and generic OpenAI-compatible inputs remain strict. Focused coverage passes for the CL-7367 runtime path.