Remove scvi-tools dependency

[Zethson]

• Replaces the existing scvi-tools implementation of the MLPClassifier with a pure Jax/Flax implementation
• Removes the mandatory scvi-tools dependency and gates Scgen behind an import lookup

[codecov-commenter]

Codecov Report

❌ Patch coverage is 79.47368% with 39 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.74%. Comparing base (ea82c20) to head (f35aa79).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
.../_perturbation_space/_discriminator_classifiers.py	81.03%	33 Missing ⚠️
pertpy/tools/__init__.py	62.50%	3 Missing ⚠️
pertpy/tools/_perturbation_space/_simple.py	62.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #844      +/-   ##
==========================================
- Coverage   72.98%   72.74%   -0.25%     
==========================================
  Files          47       47              
  Lines        5474     5510      +36     
==========================================
+ Hits         3995     4008      +13     
- Misses       1479     1502      +23     

Files with missing lines	Coverage Δ
pertpy/tools/_perturbation_space/_comparison.py	100.00% <ø> (ø)
...y/tools/_perturbation_space/_perturbation_space.py	87.43% <ø> (-0.07%)	⬇️
pertpy/tools/__init__.py	77.77% <62.50%> (-1.54%)	⬇️
pertpy/tools/_perturbation_space/_simple.py	75.60% <62.50%> (-0.03%)	⬇️
.../_perturbation_space/_discriminator_classifiers.py	81.97% <81.03%> (-7.40%)	⬇️

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