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

[karthiknadig]

Add 21 new unit tests to pet-homebrew (31 total, up from 10):

• environments.rs: version extraction (3), get_prefix always None (1), get_python_info across all prefix types (4)
• sym_links.rs: is_homebrew_python edge cases (2), get_known_symlinks_impl for all 3 prefix types (3), version regex mismatch (1), linuxbrew expected paths (1)
• lib.rs: try_from for opt/homebrew (1), usr/local/Cellar (1), conda rejection via parent/grandparent/prefix (3)
• environment_locations.rs: no-env-var behavior (1), dedup validation (1)

All tests properly gated with #[cfg(all(test, unix))]\ for platform safety.

Fixes #389

[github-actions]

Test Coverage Report (Linux)

Metric	Value
Current Coverage	76.5%
Base Branch Coverage	75.6%
Delta	.9% ✅

---

Coverage increased! Great work!

[github-actions]

Performance Report (Linux) ➖

Metric	PR (P50)	PR (P95)	Baseline (P50)	Delta	Change
Server Startup	1ms	1ms	0ms	1ms	N/A%
Full Refresh	57ms	322ms	53ms	4ms	0%

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 (Windows) ✅

Metric	PR (P50)	PR (P95)	Baseline (P50)	Delta	Change
Server Startup	9ms	12ms	11ms	-2ms	-18.2%
Full Refresh	146ms	12775ms	202ms	-56ms	-27.7%

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	91ms	639ms	85ms	6ms
Full Refresh	152ms	39048ms	175ms	-23ms

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	72.11%
Base Branch Coverage	72.49%
Delta	-0.38% ❌

---

Coverage decreased. Please add tests for new code.