Add chat attachment support: surface metadata and download images/files

Previously, get_messages and search_messages completely ignored the
attachment field on Chat API messages. This adds:

- Attachment metadata (filename, type) displayed inline in get_messages
  and search_messages output
- New download_chat_attachment tool that downloads attachments via the
  Chat API media endpoint and saves to local disk

The download uses httpx with a Bearer token against the
chat.googleapis.com/v1/media endpoint (with alt=media), which works
correctly in both OAuth 2.0 and OAuth 2.1 modes. The attachment's
downloadUri field is intentionally ignored as it points to
chat.google.com which requires browser session cookies.

Key details:
- Uses attachmentDataRef.resourceName for the media endpoint URL
- No new OAuth scopes required (existing chat_read is sufficient)
- Tool registered in the extended tier
- 10 unit tests covering metadata display, download, and edge cases

core/tool_tiers.yaml

@@ -101,6 +101,7 @@ chat:
     - search_messages
   extended:
     - list_spaces
+    - download_chat_attachment
   complete: []
 
 forms: