fix(session): extract tool-result media for models without attachment capability

[Qiiks]

Issue for this PR

Closes #41162

Type of change

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

What does this PR do?

supportsMediaInToolResult in message-v2.ts returned true unconditionally for @ai-sdk/anthropic and @ai-sdk/openai npm packages. For models with attachment: false (no vision — e.g. GLM-5.2 served through an Anthropic-compatible endpoint), image/PDF attachments from tool results stayed inside the tool result and were re-sent on every subsequent turn as part of history replay. The provider then rejected the entire request with a 400 (e.g. Model ... does not appear to support image inputs.), killing the response instead of degrading gracefully.

The fix makes supportsMediaInToolResult return false when model.capabilities.attachment is false. Media then routes to the user-message path where unsupportedParts() degrades it to a graceful text note ("ERROR: Cannot read image ... Inform the user.") and the model replies normally — matching the behavior of the openai-compatible path.

How did you verify your code works?

• Updated the existing tool-result-attachment test to assert extraction for a no-attachment model (@ai-sdk/openai, attachment: false).
• New test extracts tool-result media for anthropic models without attachment capability — anthropic npm + attachment: false → media extracted out of tool result into a user message.
• Full message-v2.test.ts suite: 40 pass / 0 fail. bun run typecheck clean.
• Reproduced the original failure in a real session (tool screenshot in history → 400 on later turn), confirmed the fix routes media to the graceful path.

Screenshots / recordings

N/A (no UI change)

Checklist

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

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