fix: validate SA3 extraction mode early

[samay2504]

This PR fixes the SA3 extraction error path by validating the extraction mode before importing optional clustering dependencies, so unsupported values now raise the expected ValueError immediately instead of being masked by an ImportError when fast_hdbscan is unavailable; this keeps user-facing errors accurate and aligns runtime behaviour with the existing test expectation.

[Copilot]

Pull request overview

This PR fixes SA3 cluster extraction error handling by validating the extraction mode before importing optional fast_hdbscan dependencies, ensuring invalid modes raise the intended ValueError instead of being masked by an ImportError.

Changes:

• Added early validation for extraction values ("eom" / "leaf") in extract_clusters.
• Removed the now-unreachable fallback else branch for invalid extraction after extraction selection.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.9%. Comparing base (9b36063) to head (e9322c4).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #516   +/-   ##
=====================================
  Coverage   76.9%   76.9%           
=====================================
  Files         29      29           
  Lines       3700    3701    +1     
=====================================
+ Hits        2846    2847    +1     
  Misses       854     854           

Files with missing lines	Coverage Δ
spopt/region/sa3.py	92.3% <100.0%> (+0.1%)	⬆️

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[martinfleis]

How does it matter which error bubbles up first? You need both - correct method and dependency.

[samay2504]

Good point, thanks. The reason I changed the order is that extraction validation is independent of optional deps, so a typo in extraction currently surfaces as a dependency error and can send users in the wrong direction. My intent was to fail fast on invalid user input first, then only check optional deps for valid extraction paths. That said, I can absolutely switch this if you prefer dependency-first error bubbling here. Happy to update to your preferred behaviour.

[martinfleis]

It simply does not matter. If you make a typo, fixing it does not help you if you don't have the dependencies. And there's no path in SA3 that would avoid using them.