Fix serial-mode regression in pickle cache file manipulation tests

[Copilot]

• Identify the regression: isolated_cache_directory fixture patches _global_params.cache_dir for all pickle/maxage tests even in serial mode, breaking tests that use module-level hardcoded paths
• Fix: only apply cache isolation when running in parallel mode (xdist), not in serial mode
• Apply ruff format to fix pre-commit.ci formatting error

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[shaypal5]

@copilot fix pre-commit.ci errors:

fix end of files.........................................................Passed
trim trailing whitespace.................................................Passed
check json...........................................(no files to check)Skipped
check yaml...............................................................Passed
check toml...............................................................Passed
check docstring is first.................................................Passed
check that executables have shebangs.....................................Passed
check for case conflicts.................................................Passed
check for added large files..............................................Passed
detect private key.......................................................Passed
typos....................................................................Passed
docformatter.............................................................Passed
mdformat.................................................................Passed
prettier.................................................................Passed
Black by Ruff............................................................Failed
- hook id: ruff-format
- files were modified by this hook

warning: `incorrect-blank-line-before-class` (D203) and `no-blank-line-before-class` (D211) are incompatible. Ignoring `incorrect-blank-line-before-class`.
warning: `multi-line-summary-first-line` (D212) and `multi-line-summary-second-line` (D213) are incompatible. Ignoring `multi-line-summary-second-line`.
1 file reformatted, 64 files left unchanged

Ruff check...............................................................Passed
mypy.....................................................................Passed
pyproject-fmt............................................................Passed
Validate pyproject.toml..................................................Passed

[shaypal5]

Excellent work identifying the problem and applying a focused fix.

[Copilot]

@copilot fix pre-commit.ci errors:

fix end of files.........................................................Passed

...

Fixed in commit 12ac8e6 — applied ruff format to collapse the multi-line if condition that pre-commit reformatted.