Adding custom test quantities to more diagnostics

[sjedhoff]

Previously, only calibration_ecdf supported custom test quantities. This PR adds that capability to additional diagnostic metrics and plotting functions.

Updated Metrics:

• calibration_error
• calibration_log_gamma
• posterior_contraction
• rott_mean_squared_error

Updated Plots:

• calibration_histogram
• coverage
• recovery
• z_score_contraction

No breaking changes; existing behavior is preserved when no custom test quantities are provided.

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
bayesflow/diagnostics/plots/recovery.py	28.57%	5 Missing ⚠️

Files with missing lines	Coverage Δ
bayesflow/diagnostics/metrics/calibration_error.py	100.00% <100.00%> (ø)
...sflow/diagnostics/metrics/calibration_log_gamma.py	100.00% <100.00%> (ø)
...sflow/diagnostics/metrics/posterior_contraction.py	100.00% <100.00%> (ø)
...low/diagnostics/metrics/root_mean_squared_error.py	60.00% <100.00%> (+7.05%)	⬆️
...yesflow/diagnostics/plots/calibration_histogram.py	92.50% <100.00%> (+1.59%)	⬆️
bayesflow/diagnostics/plots/coverage.py	97.61% <100.00%> (+0.11%)	⬆️
bayesflow/diagnostics/plots/z_score_contraction.py	96.66% <100.00%> (+1.01%)	⬆️
bayesflow/diagnostics/plots/recovery.py	82.05% <28.57%> (-11.70%)	⬇️

... and 2 files with indirect coverage changes

[paul-buerkner]

Looks good. Thank you!