fix(blocks): validate email recipients in Gmail blocks before API call (#12546)

### Why / What / How

**Why:** When a user or LLM supplies a malformed recipient string (e.g.
a bare username, a JSON blob, or an empty value) to `GmailSendBlock`,
`GmailCreateDraftBlock`, or any reply block, the Gmail API returns an
opaque `HttpError 400: "Invalid To header"`. This surfaces as a
`BlockUnknownError` with no actionable guidance, making it impossible
for the LLM to self-correct. (Fixes #11954)

**What:** Adds a lightweight `validate_email_recipients()` function that
checks every recipient against a simplified RFC 5322 pattern
(`[email protected]`) and raises a clear `ValueError` listing all invalid
entries before any API call is made.

**How:** The validation is called in two shared code paths —
`create_mime_message()` (used by send and draft blocks) and
`_build_reply_message()` (used by reply blocks) — so all Gmail blocks
that compose outgoing email benefit from it with zero per-block changes.
The regex is intentionally permissive (any `[email protected]` passes) to avoid
false positives on unusual but valid addresses.

### Changes 🏗️

- Added `validate_email_recipients()` helper in `gmail.py` with a
compiled regex
- Hooked validation into `create_mime_message()` for `to`, `cc`, and
`bcc` fields
- Hooked validation into `_build_reply_message()` for reply/draft-reply
blocks
- Added `TestValidateEmailRecipients` test class covering valid,
invalid, mixed, empty, JSON-string, and field-name scenarios

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
- [x] Verified `validate_email_recipients` correctly accepts valid
emails (`[email protected]`, `[email protected]`, `[email protected]`)
- [x] Verified it rejects malformed entries (bare names, missing domain
dot, empty strings, JSON strings)
- [x] Verified error messages include the field name and all invalid
entries
  - [x] Verified empty recipient lists pass without error
  - [x] Confirmed `gmail.py` and test file parse correctly (AST check)

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Co-authored-by: Zamil Majdy <[email protected]>