Fall back when sqlite cache is unavailable

[cyphercodes]

Fixes #21440

When the sqlite cache is enabled but the sqlite3 module cannot import (for example when CPython was built without _sqlite3), fall back to the filesystem metadata cache instead of crashing during metastore creation.

Tests:

• python3 -m pytest -q mypy/test/testmetastore.py
• python3 -m mypy --config-file mypy_self_check.ini mypy/build.py mypy/test/testmetastore.py
• pre-commit run --files mypy/build.py mypy/test/testmetastore.py
• git diff --check

[cyphercodes]

Updated this PR to make the sqlite fallback regression test work under mypyc-compiled CI.

Root cause: the test patched mypy.metastore.connect_db, which is not reliable in the mypyc-compiled test job, so the sqlite store was still constructed. The test now patches mypy.build.SqliteMetadataStore directly to raise ModuleNotFoundError, exercising the fallback path without depending on patching internals of a compiled module.

Local verification:

• python3 -m pytest mypy/test/testmetastore.py -q ✅
• git diff --check ✅

[cyphercodes]

Follow-up: the first test-only fix still failed in mypyc-compiled jobs because unittest.mock.patch does not reliably replace compiled mypy globals there. I pushed a second small test-only commit that skips this mock-based unit test for compiled mypy, matching the existing pattern used elsewhere in the test suite for source-only assertions.

Local verification:

• python3 -m pytest mypy/test/testmetastore.py -q ✅
• git diff --check ✅

CI has restarted on commit 1244b63.

[github-actions]

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