test: add unit tests for pet-global-virtualenvs crate (Fixes #389)

[karthiknadig]

Add 6 new unit tests to pet-global-virtualenvs (10 total, up from 4):

• get_global_virtualenv_dirs: all None returns empty, nonexistent work_on_home skipped, nonexistent xdg skipped
• list_global_virtual_envs_paths: all None returns empty, nonexistent virtual_env skipped, empty dirs produce no envs

Fixes #389

[github-actions]

Performance Report (Linux) ➖

Metric	PR (P50)	PR (P95)	Baseline (P50)	Delta	Change
Server Startup	1ms	1ms	0ms	1ms	N/A%
Full Refresh	64ms	335ms	53ms	11ms	20.0%

Results based on 10 iterations. P50 = median, P95 = 95th percentile.

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Legend

• 🚀 Significant speedup (>100ms faster)
• ✅ Faster than baseline
• ➖ No significant change
• 🔺 Slower than baseline (>100ms)
• ⚠️ Significant slowdown (>500ms)

[github-actions]

Test Coverage Report (Linux)

Metric	Value
Current Coverage	75.7%
Base Branch Coverage	75.6%
Delta	.1% ✅

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Coverage increased! Great work!

[github-actions]

Performance Report (macOS)

Metric	PR (P50)	PR (P95)	Baseline (P50)	Delta
Server Startup	55ms	500ms	85ms	-30ms
Full Refresh	98ms	27185ms	175ms	-77ms

Results based on 10 iterations. P50 = median, P95 = 95th percentile.

---

Legend

• 🚀 Significant speedup (>100ms faster)
• ✅ Faster than baseline
• ➖ No significant change
• 🔺 Slower than baseline (>100ms)
• ⚠️ Significant slowdown (>500ms)

[github-actions]

Test Coverage Report (Windows)

Metric	Value
Current Coverage	72.58%
Base Branch Coverage	72.49%
Delta	0.09% ✅

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Coverage increased! Great work!

[github-actions]

Performance Report (Windows) ✅

Metric	PR (P50)	PR (P95)	Baseline (P50)	Delta	Change
Server Startup	11ms	15ms	11ms	0ms	0%
Full Refresh	181ms	1861ms	202ms	-21ms	-10.4%

Results based on 10 iterations. P50 = median, P95 = 95th percentile.

---

Legend

• 🚀 Significant speedup (>100ms faster)
• ✅ Faster than baseline
• ➖ No significant change
• 🔺 Slower than baseline (>100ms)
• ⚠️ Significant slowdown (>500ms)