Gmail search: message-level retrieval unreliable even with exact sender and date (adjacent to #730)

## Summary

Given a specific sender address and an exact date, `search_threads` frequently fails to surface the target message. What should be single-query retrieval becomes a multi-turn guessing session that can still fail.

## Reproduction

Request: "Find the email from billing@ received Monday 10 Aug around 14:00." Four query formulations were tried — `from:` with the exact address, bare keywords, `newer:`, and an `after:`/`before:` window. The correct thread was found repeatedly; the specific Aug 10 message never appeared in any returned message list, despite every identifying detail being known.

## Relationship to #730

The thread-matched-but-message-missing half may share a root with #730's capped 5-message snapshot — if a matched thread returns only a capped message list, a mid-thread message is invisible no matter how precise the query. Filed separately because the rest is distinct:

- **No message-level search.** Results are threads; a thread can match on *other* messages in it. A message-level tool taking structured fields (sender, date range, subject keywords) would fit the "find this email" task directly.
- **No exact-date guidance.** `after:`/`before:` exist, but nothing documents how to target a single day; Claude constructs bracket queries by guesswork.
- **No fuzzy sender matching.** "billing at " must be hand-translated to exact operator syntax; one malformed query (a space in the domain) silently returned nothing. Approximate sender matching would remove that failure class.

## Impact

"Find this email" is the connector's most common task, and it fails often enough to erode trust in the tool as a retrieval surface.