BYOK PDF support requires a hardcoded model family

VS Code 1.128 gates PDF attachment support through `modelSupportsPDFDocuments`, which falls back to an Anthropic/GPT family allowlist when no explicit endpoint capability exists.

For custom BYOK endpoints, the family is derived from the configured model ID. A compatible proxy or another model that accepts PDF documents therefore has to use an allowlisted-looking ID (or a global family capability override) to unlock PDFs. Family overrides affect many unrelated prompt and capability heuristics. Models outside the allowlist cannot advertise PDF support independently.

It would be useful for each language model endpoint to explicitly advertise accepted file MIME types, for example `fileInputMimeTypes: ['application/pdf']`, with the existing family check retained as a compatibility fallback.

#324960 fixes the separate Anthropic Messages API conversion bug but intentionally does not change capability advertisement or the public API surface. The broader approach was previously explored in #323836.