fix: gate non-media files to prevent converter crash

[jwcrystal]

Issue for this PR

Closes #35697

Type of change

• Bug fix

What does this PR do?

The server gate in message-v2.ts pushes any file part through to provider converters regardless of media type. When a client sends a file with unrecognized MIME type (application/octet-stream), the converter can only handle image/* and throws UnsupportedFunctionalityError, crashing the session.

This was exposed by the custom provider image support change that set image capability to true — non-image files started flowing through when previously they were filtered out.

Fix: invert the gate condition so only actual media (image/*, application/pdf) gets pushed as a file part. Everything else becomes a text placeholder. Both copilot converters also silently skip unrecognized file types as defense-in-depth.

How did you verify your code works?

• Ran cd packages/core && bun test test/github-copilot/ — 37/37 pass
• Ran cd packages/opencode && bun test test/session/message-v2.test.ts — 36/36 pass
• Ran bun turbo typecheck — 30/30 packages clean

Screenshots / recordings

N/A — logic change only

Checklist

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

[github-actions]

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