fix: remove Anthropic Files API usage

[rumpl]

Summary

• remove Anthropic Files API upload/delete manager and tests
• stop adding the files-api beta header or routing legacy file parts to Anthropic file IDs
• keep legacy message part structs for stored-session compatibility while documenting that new attachments should use documents

Tests

• go test ./pkg/model/provider/anthropic ./pkg/chat
• MOONSHOT_API_KEY= task test
• task lint
• task build

[docker-agent]

Assessment: 🟢 APPROVE

The PR cleanly removes the Anthropic Files API (FileManager, upload/delete logic, the files-api beta header, and the routing guard). Legacy MessagePartTypeFile structs are retained for stored-session deserialization compatibility — a good design choice. Two minor observability gaps were found in the new MessagePartTypeFile handling paths.

[docker-agent]

[LOW] Legacy FileID-only file parts silently dropped with no warning in non-beta path

When a stored session contains a MessagePartTypeFile part where part.File == nil (i.e., only a FileID string is present, no local file object), this case reaches the convertUserMultiContent non-beta path and silently skips the part with no log entry. The part.File != nil branch correctly logs a deprecation warning, but the FileID-only case goes unlogged.

Consider also warning when part.File == nil (e.g. checking part.FileID != ""):

case chat.MessagePartTypeFile:
    if part.File != nil {
        slog.WarnContext(ctx, "anthropic: dropping legacy file attachment; use document attachments instead", "path", part.File.Path)
    } else if part.FileID != "" {
        slog.WarnContext(ctx, "anthropic: dropping legacy Files-API file ID; use document attachments instead", "file_id", part.FileID)
    }

[docker-agent]

[LOW] Beta path silently drops legacy file parts with no log warning (inconsistent with non-beta path)

The non-beta path (convertUserMultiContent) logs a slog.WarnContext deprecation message when it encounters a MessagePartTypeFile part, but the beta path here uses a bare continue with no log output. A session that reaches createBetaStream (e.g. because it uses structured output or extended thinking) with legacy file parts will silently lose those attachments with no operator-visible signal.

For consistency, add the same warning here:

case chat.MessagePartTypeFile:
    if part.File != nil {
        slog.WarnContext(ctx, "anthropic: dropping legacy file attachment; use document attachments instead", "path", part.File.Path)
    }
    continue