Add passthrough mode to GaussianSmoothingFilterTransformer

[cboulay]

Summary

• GaussianSmoothingFilterTransformer now acts as a passthrough (no smoothing) when settings describe a degenerate kernel: sigma is None or <= 0, width is None or <= 0, or kernel_size is provided and <= 1.
• The check lives in the design wrapper, which returns None coefficients — rather than returning None from get_design_function — so passthrough works on both the sync __call__ path and the async unit path (__acall__ → _reset_state, which calls design_fun(fs) without a None check). Downstream, FilterTransformer with coefs=None already returns messages untouched, matching the established Butterworth order=0 pattern.
• Smoothing can be disabled and re-enabled live via update_settings.

Behavior change

kernel_size=1 on the transformer previously designed a single-tap identity kernel with a UserWarning; it is now a silent passthrough. The standalone gaussian_smoothing_filter_design function is unchanged and still raises ValueError on invalid inputs (and warns on an identity kernel).

Tests

• test_gaussian_passthrough_settings: parametrized over all nine degenerate settings; asserts identity passthrough with no warnings and coefs is None on the inner filter.
• test_gaussian_passthrough_async_path: drives __acall__ directly to guard the pipeline path that bypasses the sync shortcut.
• test_gaussian_passthrough_toggle: disable → enable → disable via update_settings.
• Replaced the old kernel_size=1 warning test with test_gaussian_identity_kernel_design_warns for the standalone design function.

Full unit suite passes: 3593 passed, 5 skipped.

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