fix(gateway): preserve update notification retry state

[mths-mcfs]

Summary

• Treat SendResult(success=False) as a failed /update completion notification delivery.
• Preserve .update_pending*, .update_output.txt, and .update_exit_code so the gateway can retry instead of dropping the final update result.
• Add regression coverage for both the streaming watcher and the post-restart fallback notification path.

Background

PR #39091 fixed the missing/offline adapter case for /update completion notifications. This PR covers the adjacent soft-failure case where an adapter exists and adapter.send(...) returns SendResult(success=False) without raising.

Sensitive data audit

• The PR contains only changes to gateway/run.py and two gateway update tests.
• Test chat/user IDs are dummy values (111 and 222).
• A staged-diff scan checked for common token/key/secret patterns, private key blocks, JWTs, Telegram bot tokens, absolute local home paths, likely real Telegram chat IDs, and e-mail addresses.
• Commit metadata uses a GitHub noreply address.

Test Plan

• uv run --extra dev pytest tests/gateway/test_update_streaming.py tests/gateway/test_update_command.py -o 'addopts=' -q
• uv run --extra dev python -m py_compile gateway/run.py tests/gateway/test_update_streaming.py tests/gateway/test_update_command.py
• uv run --extra dev ruff check gateway/run.py tests/gateway/test_update_streaming.py tests/gateway/test_update_command.py
• uv run --extra dev pytest tests/gateway/test_platform_reconnect.py -o 'addopts=' -q

[teknium1]

Thanks for covering the SendResult(success=False) path. Current main still discards the return value of the final streaming send at gateway/run.py:14602-14612 and of the post-restart send at gateway/run.py:14806-14810, followed by marker cleanup at gateway/run.py:14617-14623 and gateway/run.py:14819-14824. gateway/platforms/base.py:1878-1892 defines SendResult.success as the delivery outcome, so the proposed preservation behavior addresses a real current-main bug in both delivery paths.

Automated hermes-sweeper review.