Feature Request: Built-in PDF Reading Support

### Describe the feature or problem you'd like to solve

Currently, Copilot CLI cannot natively read PDF documents, which limits its ability to help users working with academic papers, technical documentation, reports, and other PDF-based content. Users must manually install tools like pdftotext (via poppler) or Python libraries like PyPDF2 to work with PDFs.

### Proposed solution

Integrate native PDF reading capabilities into GitHub Copilot CLI to enable direct analysis of PDF documents without requiring users to manually install external dependencies.

Built-in PDF Parser
- Bundle a lightweight PDF parsing library
- Automatically detect PDF files and extract text content when users reference them
- Handle common PDF formats and return structured text output

### Example prompts or workflows

_No response_

### Additional context

_No response_