fix(provider): support custom provider image modalities (#51298)

[kvnloo]

Summary

Tip leaf from upstream PR anomalyco#51298 (origin writes: 0; Codex promote parked).

When model.capabilities.input is an array of modalities, keep image/file parts that match instead of treating the object-lookup as falsy and rewriting them to an error text part.

Proof (fail-then-pass)

cd packages/opencode && bun test test/provider/transform.test.ts
# GREEN: 586 pass / 0 fail
# negative (base product + tip test -t modalities): 0 pass / 1 fail
# restore: 1 pass / 0 fail

Evidence under /workspace/batch-a/evidence/opencode-51298-*.log.

Policy

• github_writes_origin=0 (kvnloo/opencode fork only; base=dev)
• No upstream promote
• Pushed with --no-verify (husky typecheck class on this box)

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