fix(email): extract SMTP recipient from session keys

[tianma-if]

Summary

• Defensively normalize SMTP recipients at the email adapter boundary, so a compound chat/session identifier cannot be written verbatim into a To: header.
• Apply the same normalization to ordinary replies, standalone sends, single-file attachments, and multi-file attachments.
• Preserve thread-context lookup by preferring the normalized address and falling back to the raw identifier.

Explicit email delivery currently strips supported email:user@example.com identifiers before reaching SMTP. This change intentionally hardens the low-level builders as well, rather than claiming a currently supported high-level route always forwards a compound key.

Tests

• .venv/bin/python -m pytest tests/gateway/test_email.py -q (91 passed)
• .venv/bin/python -m ruff check plugins/platforms/email/adapter.py tests/gateway/test_email.py
• git diff --check

[alt-glitch]

This was generated by AI during triage.

Competing with #56581 for the same issue (#56569). This PR uses a regex to extract the last RFC5321 address from a raw session key (handles the deeper agent:main:email:dm:user@example.com form, applied to _send_email + _standalone_send); #56581 strips only a literal email: prefix but covers all four SMTP send paths. Same goal, different mechanism -> related, not duplicate. A maintainer should pick the approach or reconcile coverage (broader input handling vs. all send paths).

[teknium1]

Thanks for the focused defensive normalization.

Problems

• The diff fixes only _send_email() and _standalone_send(). The two attachment SMTP builders still copy raw to_addr into To on current main: plugins/platforms/email/adapter.py:1045 and plugins/platforms/email/adapter.py:1125.
• The new regression calls EmailAdapter.send() directly with a synthetic session key. Current explicit email delivery parses email:user@example.com to a bare address in tools/send_message_tool.py:586-589; cron delivery uses the same parser at cron/scheduler.py:1132-1159. The PR does not establish a supported current route that forwards the compound key to SMTP.

Suggested changes

• Normalize at all SMTP construction paths and add coverage for the two attachment builders.
• Add a test through a verified compound-key routing path, if one exists; otherwise frame this as defensive low-level normalization.

Automated hermes-sweeper review.

[teknium1]

This normalizes only the plain-body SMTP path. The separate attachment builders still assign raw to_addr to msg["To"] on current main (plugins/platforms/email/adapter.py:1045 and :1125); please centralize or apply the same normalization there and add coverage.