Refactor vector field potentials: add init() setup and PosteriorBuilder

[rogueslasher]

Summary

Introduce an explicit init() method for potentials to allow one-time setup
instead of relying on runtime caching.

Changes

• Added no-op init() to BasePotential
• Implemented init() in VectorFieldBasedPotential to warm iid/guidance caches
• Added PosteriorBuilder with with_iid() and with_guidance()
• Updated VectorFieldPosterior to support builder pattern
• Added tests for PosteriorBuilder and init() separation

Tests

All tests pass locally. One test is marked xfail intentionally because the
behavior is expected to fail by design.

Closes #1783

[codecov]

Codecov Report

❌ Patch coverage is 25.45455% with 41 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@7d66b89). Learn more about missing BASE report.
⚠️ Report is 7 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
sbi/inference/posteriors/vector_field_posterior.py	27.02%	27 Missing ⚠️
sbi/inference/potentials/vector_field_potential.py	6.66%	14 Missing ⚠️

❌ Your patch check has failed because the patch coverage (25.45%) is below the target coverage (50.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1802   +/-   ##
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  Coverage        ?   86.70%           
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  Files           ?      142           
  Lines           ?    13125           
  Branches        ?        0           
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  Hits            ?    11380           
  Misses          ?     1745           
  Partials        ?        0           

Flag	Coverage Δ
fast	82.47% <25.45%> (?)

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Files with missing lines	Coverage Δ
sbi/inference/potentials/base_potential.py	93.10% <100.00%> (ø)
sbi/inference/potentials/vector_field_potential.py	81.75% <6.66%> (ø)
sbi/inference/posteriors/vector_field_posterior.py	66.66% <27.02%> (ø)

[rogueslasher]

The codecov patch failure is expected. The new code paths PosteriorBuilder.sample(), PosteriorBuilder.log_prob(), and VectorFieldBasedPotential.init() are all covered by the slow tests in tests/test_posterior_builder.py which require a trained NPSE model. These are marked @pytest.mark.slow and don't run in CI's fast test suite.The fast test test_base_potential_init_is_noop covers the BasePotential change (93.1%). Happy to add mock-based fast tests for the remaining lines if preferred.