feat: add fetch_pdf tool

[meirk-brd]

Summary:

• Introduce fetch_pdf tool that streams PDFs via Bright Data's unblocker with size caps and extracts text using pdf-parse, guarded by a shared p-limit to prevent memory spikes.
• Add pdf-parse and p-limit runtime dependencies.
• Harden PDF handling with try/catch around streaming and parsing; surface clean errors to clients.

Testing:

• Invoke fetch_pdf against arXiv PDF and observe successful error reporting without a process crash.