fix(provider): respect attachment image capability

[manuelfedele]

Issue for this PR

Closes #33542

Type of change

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

What does this PR do?

Custom provider models configured with attachment: true were still marked as not accepting image input unless modalities was also provided. OpenCode then replaced the image with an error before building the provider request.

This preserves the existing precedence: explicit modalities.input wins, then discovered model metadata, then attachment: true enables image input for custom model configuration.

How did you verify your code works?

• bun test test/provider/provider.test.ts --timeout 30000 from packages/opencode
• bun typecheck from packages/opencode
• bun test test/config.test.tsx --timeout 30000 from packages/tui
• TURBO_CONCURRENCY=1 bun typecheck from the repository root
• The pre-push hook passed with TURBO_CONCURRENCY=1

Screenshots / recordings

N/A. This is a provider capability fix with no visual change.

Checklist

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