Don't create .mypy_cache directory when --no-incremental is used

[Fridayai700]

Summary

When running mypy with --no-incremental, the .mypy_cache directory is still created, even though the help text implies it won't be:

$ rm -rf .mypy_cache
$ mypy t.py --no-incremental
Success: no issues found in 1 source file
$ ls -d .mypy_cache
.mypy_cache  # Unexpected!

Root cause

The metadata store (which manages the cache directory) is initialized unconditionally in BuildManager.__init__(). While the cache_enabled flag prevents cache reads, the store still creates the cache directory on init (for SQLite stores) or on first write (for filesystem stores).

Fix

When options.incremental is False, create_metastore() now returns a no-op FilesystemMetadataStore backed by os.devnull. Both store implementations already handle os.devnull as a no-op — this is the same behavior users get with --cache-dir /dev/null, which was the documented workaround.

After:

$ rm -rf .mypy_cache
$ mypy t.py --no-incremental
Success: no issues found in 1 source file
$ ls -d .mypy_cache
ls: cannot access '.mypy_cache': No such file or directory  # Correct!

Test plan

• Manual verification: --no-incremental no longer creates .mypy_cache
• Normal mode still creates cache as expected
• All 147 incremental-related tests pass
• python -m pytest mypy/test/testcheck.py -k "testNoIncremental or testIncremental" -o "addopts=" — all pass

Fixes #19489