fix(email): enable splits_long_messages to avoid 4000-char truncation cap

[liuhao1024]

What does this PR do?

Adds splits_long_messages = True to EmailAdapter class attributes.

The delivery router (gateway/delivery.py) checks adapter.splits_long_messages at line 431 to decide whether to truncate content at MAX_PLATFORM_OUTPUT (4000 chars) or deliver the full payload. When True, the full content passes to the adapter's send() method, which can chunk natively. When False (the default), content above the cap is truncated with a footer pointing to a saved file.

EmailAdapter was missing this declaration, causing all email delivery to hit the truncation path even though:

• SMTP has no practical message length limit at 4000 chars
• The adapter's send() method handles the full payload via MIMEText(body, "plain", "utf-8")
• The adapter internally defines MAX_MESSAGE_LENGTH = 50_000, indicating intent to support much longer messages

All other platform adapters (Telegram, Discord, Slack, Matrix, WhatsApp, etc.) already declare splits_long_messages = True. This change aligns email with their behavior and fixes the silent truncation bug where cron job output was cut off mid-content.

Related Issue

Fixes #61990

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• plugins/platforms/email/adapter.py: Added splits_long_messages = True class attribute with explanatory comment
• tests/gateway/test_email.py: Added TestEmailAdapterSplitsLongMessages test class to verify the attribute is declared

How to Test

1. Verify the test passes: pytest tests/gateway/test_email.py::TestEmailAdapterSplitsLongMessages -v
2. Check that the adapter declares the attribute: python -c "from plugins.platforms.email.adapter import EmailAdapter; print(EmailAdapter.splits_long_messages)" should print True
3. (Integration test) Configure email gateway adapter, create a cron job that generates output longer than 4000 characters (e.g., a daily briefing), and verify the received email contains the full output without truncation

Observed result: The test passes, confirming that EmailAdapter.splits_long_messages is True.

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: macOS 26.5.1 (Apple Silicon)

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

[teknium1]

Thanks for isolating the missing capability declaration. The current-head premise is valid: gateway/delivery.py:431 truncates when the flag is false, while EmailAdapter.send() forwards its body to MIMEText and smtp.send_message at plugins/platforms/email/adapter.py:893-954.

Problems

• tests/gateway/test_email.py:1778 asserts only the implementation attribute. It does not exercise the reported delivery contract. The existing full-payload router coverage is generic (tests/gateway/test_delivery.py:334-351), not email-specific.
• The base contract says splits_long_messages means send() splits through truncate_message() (gateway/platforms/base.py:2290-2296), but email sends one full SMTP body (plugins/platforms/email/adapter.py:949-954). The new comment should not call that chunking.

Suggested changes

• Add a router-level >4000-character email regression test that captures EmailAdapter.send() input and asserts it equals the original content.
• Describe email as preserving a native long message, or broaden the capability documentation accordingly.

Automated hermes-sweeper review.

[teknium1]

splits_long_messages is documented as send() splitting through truncate_message() in gateway/platforms/base.py:2290-2296, but this adapter sends one MIMEText body via SMTP. Please describe this as native full-message delivery (and consider broadening the base capability contract) rather than native chunking.

[teknium1]

Please add a router-level regression using this adapter (with send captured) and a payload over MAX_PLATFORM_OUTPUT; asserting the class attribute alone does not prove that the email delivery route receives the full body.