fix(server): handle OpenCode text response format in commit message g…

[imabdulazeez]

What Changed

• Updated the OpenCode git text-generation path to read JSON from text parts in the prompt response instead of requiring info.structured.
• Surface OpenCode's returned structured-output error message when the provider reports one, and keep failing fast on empty responses.
• Reused the existing JSON-object extraction logic from the Cursor implementation and moved it into apps/server/src/git/Utils.ts as shared logic so OpenCode and Cursor parse provider output the same way.
• Updated apps/server/src/git/Layers/OpenCodeTextGeneration.test.ts to cover the failing OpenCode response shapes: empty output, JSON returned inside plain text parts, and upstream structured-output errors.
• Tested both OpenCode and Cursor for git text generation to confirm the shared JSON extraction path still works for each provider.

Why

OpenCode is currently returning the generated payload as plain text parts rather than populating info.structured, so the existing implementation treated valid responses as failures and broke commit message generation.

Parsing the returned text locally and decoding it against the existing schema keeps the validation path intact while matching the response shape OpenCode actually sends.

UI Changes

Before

After

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes (not applicable)

Closes #2199

Note

Fix OpenCode text response parsing in commit message generation

• Removes the json_schema structured output format from OpenCode requests; instead reads raw text parts and extracts JSON locally using extractJsonObject from Utils.ts.
• Adds getOpenCodeTextResponse to concatenate text parts into a single string, and getOpenCodePromptErrorMessage to surface upstream info.error details as thrown errors.
• Throws a typed 'OpenCode returned empty output.' error when no text parts are present, and maps info.error with StructuredOutputError to a readable message.
• Moves extractJsonObject to Utils.ts and shares it with CursorTextGeneration.ts.
• Behavioral Change: OpenCode prompts no longer request structured JSON output; JSON is parsed client-side from plain text responses.

^{Macroscope summarized 2a12331.}

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[macroscopeapp]

Approvability

Verdict: Approved

This is a focused bug fix that adapts OpenCode response parsing from structured output to text format, moving a shared utility and adding error handling helpers. Changes are limited to text generation layers with comprehensive test coverage, no runtime behavior changes beyond fixing the response format handling.

^{You can customize Macroscope's approvability policy. Learn more.}

[juliusmarminge]

cc @nexxeln is your structured data not reliable?