gmail_read_message does not return CC/BCC headers

**Description**

The `gmail_read_message` tool only returns four headers: From, To, Subject, and Date. The Cc and Bcc fields are omitted from the response, even when present on the original message.

**Impact**

Without CC/BCC headers, it's impossible to:
- Know who else is on a conversation
- Construct a proper Reply All
- Avoid accidentally excluding participants from a thread

This affects any workflow where multiple recipients are involved — which is most professional email.

**Steps to reproduce**

1. Receive an email where the sender has CC'd one or more recipients
2. Use `gmail_read_message` to read the message
3. Observe that the returned headers object contains only From, To, Subject, and Date
4. The CC'd addresses are not available anywhere in the response

**Expected behavior**

The headers object should include Cc (and ideally Bcc where available) so that reply drafts can be composed with the correct recipient list.

**Additional context**

The Gmail API itself returns these headers — they appear to be filtered out by the connector before the response is returned. Related: https://github.com/anthropics/claude-code/issues/28575 (another Gmail connector gap around draft attachments and sending).