Add PDF document support for Messages API (Anthropic Claude models)

[bhavyaus]

Summary

Add support for sending PDF documents as content parts in the Messages API, enabling native PDF attachment handling for Anthropic Claude models.

Details

This adds:

• A new Document content part kind in prompt-tsx for PDF payloads
• PDF magic byte validation (%PDF / 0x25 0x50 0x44 0x46)
• Model capability gating via modelSupportsPDFDocuments() (Anthropic-only for now)
• Conversion to Anthropic DocumentBlockParam in the Messages API layer
• Conversation history replay support for document content parts
• Token estimation heuristic for PDF documents (avoids BPE tokenization of large binary payloads)
• Graceful degradation: unsupported models show an omitted reference with a descriptive message

Implementation

PR: microsoft/vscode-copilot-chat#4473

[isidorn]

Bhavya U (@bhavyaus) I see this working, and I do see the PR is merged.

Did we just forget to close this issue?

[bhavyaus]

Yes! Closing this!