fix(gateway): convert execution-denied tool results to text

[nicoalbanese]

Background

When a user denies a tool execution approval (answers "n" to an approval prompt), the SDK creates a tool result with output: { type: 'execution-denied' }. The Gateway API doesn't recognize this type, returning GatewayInvalidRequestError: Invalid input at "prompt",3,"content",0,"output".

Other providers (OpenAI, Anthropic, Mistral, etc.) handle this by converting execution-denied to plain text in their message converters before sending to their APIs.

Summary

• Renamed maybeEncodeFileParts() → preparePrompt() to reflect its broader responsibility
• Added conversion of execution-denied tool results to { type: 'text', value: reason } before sending to Gateway API
• Added tests for the conversion

Manual Verification

1. Run examples/ai-core/src/stream-text/openai-tool-approval.ts
2. Change provider/model to use gateway
3. Ask for weather somewhere, then reject the execution approval
4. Verify no error is thrown and the model responds appropriately

Checklist

• Tests have been added / updated (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• A patch changeset for relevant packages has been added (for bug fixes / features - run pnpm changeset in the project root)
• I have reviewed this pull request (self-review)

Related Issues

Fixes #10663

[nicoalbanese]

DO NOT MERGE

@shaper looking into gateway fix here.
cc @lgrammel