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

[karthiknadig]

Add 22 new unit tests to pet-pyenv (27 total, up from 5 integration tests):

• environments.rs (14 tests): get_version parsing (stable, dev, alpha, win32, non-version, partial, rc limitation), get_generic_python_environment (version extraction, win32 arch, no-arch, manager, unrecognized folder), get_virtual_env_environment (with/without pyvenv.cfg)
• environment_locations.rs (8 tests): get_pyenv_dir (PYENV_ROOT priority, PYENV fallback, no vars), get_home_pyenv_dir (no home, expected path), get_binary_from_known_paths (found, not found, empty)

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	65ms	225ms	53ms	12ms	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	75.9%
Base Branch Coverage	75.6%
Delta	.3% ✅

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Coverage increased! Great work!

[github-actions]

Performance Report (Windows) ✅

Metric	PR (P50)	PR (P95)	Baseline (P50)	Delta	Change
Server Startup	9ms	12ms	11ms	-2ms	-18.2%
Full Refresh	147ms	401ms	202ms	-55ms	-27.2%

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]

Performance Report (macOS)

Metric	PR (P50)	PR (P95)	Baseline (P50)	Delta
Server Startup	57ms	727ms	85ms	-28ms
Full Refresh	105ms	30660ms	175ms	-70ms

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	72.93%
Base Branch Coverage	72.49%
Delta	0.44% ✅

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Coverage increased! Great work!