search_conversations tool excludes sender/recipient metadata from search

The search_conversations tool is described as performing a "full-text search query for conversations." This description does not indicate that the search index covers only message bodies and subject lines, sender, recipient, to, from, and cc fields are not searched. "Full-text search" conventionally implies that all text associated with a conversation is indexed, including metadata fields like sender and recipient addresses.

Searching for an email address contact@domain.com returns no results for threads where that address appears only in email headers (to/from/cc), even though those threads are accessible via read_conversation. The address must appear in the message body itself to be matched.

Reproduction steps:

1. Identify a conversation where contact@domain.com appears in the to or from field but not in the message body
2. Run search_conversations with query: "contact@domain.com" → returns no results
3. Fetch the conversation directly via read_conversation → confirms the conversation exists and the address is in the headers