[ENH] add isj bandwidth option to kernel base models

[NandiniDhanrale]

Summary

• add "isj" as a supported automatic bandwidth rule for KernelMixture
• extend NadarayaWatsonCDE.y_bandwidth to accept "isj" for the 1D target-kernel path
• add focused tests for the new rule and degenerate-target fallback behavior

Testing

• py -3.11 -m pytest -o addopts= skpro/regression/tests/test_bandwidth.py skpro/distributions/tests/test_kernel_mixture.py skpro/regression/tests/test_nadaraya_watson.py -q

Fixes #980

[patelchaitany]

@NandiniDhanrale You can drop this test file. params6 in get_test_params already runs the ISJ path end-to-end through the framework. The "matches helper" test calls bandwidth_1d on both sides, so it's really just checking a == a. If you want to keep the degenerate fallback check, put it in test_bandwidth.py as a unit test on bw_isj_1d.

[NandiniDhanrale]

Addressing the duplicate-coverage point:

• dropped skpro/regression/tests/test_nadaraya_watson.py
• moved the degenerate-data fallback into bw_isj_1d, so the shared helper owns that behavior
• added the focused degenerate fallback check in skpro/regression/tests/test_bandwidth.py
• kept the public-surface coverage through get_test_params / framework tests

Re-ran:

• py -3.11 -m pytest -o addopts= skpro/regression/tests/test_bandwidth.py skpro/distributions/tests/test_kernel_mixture.py -q
• py -3.11 -m pytest -o addopts= skpro/regression/tests/test_all_regressors.py -k NadarayaWatsonCDE -q