Fix dmypy suggest crash after flushed diagnostics

[vetrovk]

Fixes #21765.

dmypy suggest could reload an automatically loaded module after a prior check had flushed diagnostics. If that reload reprocessed a user module, adding a new diagnostic violated the flushed_files invariant and crashed the daemon.

Reset the suggestion reload error accumulator before the fine-grained update, matching the existing fine-grained trigger behavior. Add a daemon regression test covering stdlib suggestions after a note, repeated suggestions, daemon status, and a subsequent incremental check.

Validation:

• mypy/test/testdaemon.py
• mypy/test/testfinegrained.py -k Suggest
• mypy/test/testfinegrained.py
• python runtests.py self
• python runtests.py lint
• git diff --check

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅