…mpCacheDir

Enhanced the _TempCacheDir.__init__ method to properly respect and handle
the TMPDIR environment variable when creating temporary cache directories.

Changes:
- Add TMPDIR environment variable detection and validation
- Normalise paths to handle path resolution issues
- Auto-create TMPDIR directory if it doesn't exist to prevent silent
  fallback to default temporary directory
- Validate that TMPDIR is actually a directory before use
- Explicitly pass directory to mkdtemp to ensure TMPDIR is respected even
  if tempfile.gettempdir() was already called and cached
- Add appropriate logging for directory creation and fallback scenarios

This ensures that when TMPDIR is set, the temporary cache files are
created in the specified directory rather than silently falling back to
the system default temporary directory.

…mpCacheDir

Add test coverage for the improved TMPDIR environment variable handling
in the _TempCacheDir class. These tests verify the various scenarios
for TMPDIR usage and error handling.

Changes:
- Refactor test_fingerprint_in_multiprocessing to use Pool.map for
  cleaner test implementation
- Add test_temp_cache_dir_with_tmpdir_nonexistent to verify TMPDIR
  auto-creation when directory doesn't exist
- Add test_temp_cache_dir_with_tmpdir_existing to verify correct
  behaviour when TMPDIR exists and is valid
- Add test_temp_cache_dir_without_tmpdir to verify fallback to default
  temporary directory when TMPDIR is not set
- Add test_temp_cache_dir_tmpdir_creation_failure to verify graceful
  error handling and fallback when TMPDIR creation fails

These tests ensure that the TMPDIR improvements work correctly across
all scenarios and edge cases, including proper logging and fallback
behaviour.

Refine TMPDIR-related failure tests for _TempCacheDir to assert explicit
error conditions instead of fallback behaviour.

Changes:
- Update test_temp_cache_dir_tmpdir_creation_failure to use _TempCacheDir
  directly and assert that an OSError is raised with a clear TMPDIR
  context when directory creation fails
- Introduce test_temp_cache_dir_tmpdir_not_directory to verify that
  pointing TMPDIR at a non-directory raises an OSError with an
  informative error message

These tests better match the intended contract of _TempCacheDir by
ensuring invalid TMPDIR configurations fail loudly with descriptive
messages rather than silently falling back.

…loudly

Tighten TMPDIR handling in _TempCacheDir so that invalid configurations
raise clear errors instead of silently falling back to the default
temporary directory.

Changes:
- When TMPDIR points to a non-existent directory, raise an OSError with
  explicit guidance to create it manually or unset TMPDIR
- When TMPDIR points to a non-directory path, raise an OSError with
  guidance to point TMPDIR to a writable directory or unset it
- Remove previous warning-and-fallback behaviour to avoid masking
  configuration issues

This ensures that TMPDIR misconfigurations are surfaced early and
clearly, aligning runtime behaviour with the stricter expectations
codified in the new tests.