feat(anthropic): add PDF document support for file content blocks

[Nic-vdwalt]

Summary

The Anthropic provider's ContentTypeFile handler only supports image/* and text/* MIME types, silently dropping all other file types (including PDFs) with a // TODO: handle other file types comment. The Anthropic API has supported PDF documents via document content blocks since 2024, and the SDK already provides NewDocumentBlock and Base64PDFSourceParam.

This PR:

• Adds a case file.MediaType == "application/pdf" that creates a document block using Base64PDFSourceParam, with cache control support matching the existing image handling
• Adds block.OfDocument != nil to hasVisibleUserContent() so messages containing only document blocks aren't incorrectly dropped as "empty"
• Adds cache control support for text/* document blocks, matching the existing image and PDF behavior (was previously missing)
• Updates the existing "drop unsupported content" test to use application/zip instead of application/pdf
• Adds tests verifying PDF and text document content is retained in messages

Test plan

• go test ./providers/anthropic/... passes
• Send a PDF file via an application using the Anthropic provider and verify the model receives and responds to the PDF content

[andreynering]

Hi @Nic-vdwalt 👋

This looks good overall. I'll try to test it once I can.

That said, can you confim to me that you have tested this yourself and ensures that it works?

[Nic-vdwalt]

Hi @Nic-vdwalt 👋

This looks good overall. I'll try to test it once I can.

That said, can you confim to me that you have tested this yourself and ensures that it works?

Hi @andreynering, thanks for reaching out. Yes, it works.

[andreynering]

Thank you!