fix(providers/anthropic): forward FilePart.Filename as document title and warn on unsupported media types

[ethanndickson]

Summary

The Anthropic provider currently ignores FilePart.Filename for both PDFs and text documents, and it silently drops any other media type without surfacing a warning. Claude therefore has no handle the model can use to refer back to an attachment, and unsupported attachments leave no trace for the caller.

This PR makes three small changes to providers/anthropic:

1. PDF filename → document title. Forward file.Filename into DocumentBlockParam.Title on the application/pdf branch. The filename is sanitized first (see below).
2. Text filename → document title. Same forwarding on the text/* branch, which also produces a DocumentBlockParam (via PlainTextSourceParam).
3. Default branch with CallWarning. Match the other providers in this repo by emitting a fantasy.CallWarning when a FilePart media type is not handled, instead of silently dropping it.

docBlock.OfDocument.Title = anthropic.String(sanitizeAnthropicDocumentTitle(file.Filename))

default:
    warnings = append(warnings, fantasy.CallWarning{
        Type:    fantasy.CallWarningTypeOther,
        Message: fmt.Sprintf("file part media type %s not supported", file.MediaType),
    })

Why title and the sanitizer

Anthropic restricts document titles to alphanumerics, whitespace, hyphens, parentheses, and square brackets. A title that contains other runes (the . and _ characters that occur in almost every real filename, for example) is rejected with:

The document file name can only contain alphanumeric characters, whitespace characters, hyphens, parentheses, and square brackets.

The new sanitizeAnthropicDocumentTitle helper replaces disallowed runes with spaces, collapses consecutive whitespace, and trims. Empty or fully disallowed input falls back to "Document" so every attached document has a stable handle the model can refer back to.

Why the image branch is untouched

ImageBlockParam does not have a free-form filename or title slot, so there is no equivalent place to forward FilePart.Filename. The two branches touched here (application/pdf, text/*) are the full set of Anthropic content blocks that can carry a document title. The new default warning still covers everything else, including image MIME types that the existing image/* case does not handle (for example, image/heic).

Upstream

The same gaps exist in charmbracelet/fantasy. Tracked in charmbracelet#267.

Closes

CODAGT-540 (follow-up to #37)