feat(content-uploader): add onSelection callback (#3839)

[devin-ai-integration]

Description

This PR adds an onSelection callback to the ContentUploader component to allow developers to control file selection before upload begins. This implements the suggestion from issue #3839.

Changes

• Added onSelection prop to ContentUploader component
• Added TypeScript and Flow type definitions
• Implemented file selection validation in UploadInput component
• Added comprehensive test coverage for the new callback

Usage

<ContentUploader
  onSelection={(files) => {
    // Return false to prevent upload
    // Return true to allow upload to proceed
    return files.length <= 5; // Example: Only allow up to 5 files
  }}
  {...otherProps}
/>

Testing

• Added unit tests for UploadInput component
• Verified all tests pass
• Ran lint checks via precommit hooks

Link to Devin run: https://app.devin.ai/sessions/acbecd8da4154ad8aba8d8d81d7bd661

[devin-ai-integration]

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