[FIX] : Add substring fallback for tag suggestions in list_estimators

[Rohitstwt]

Reference Issues/PRs

Related to #35 (early adopter feedback - tag suggestion returning None for partial key matches)

What does this implement/fix?

When an invalid tag key like pred_int is passed to list_estimators_tool, the existing code returns suggestions: None even though capability:pred_int exists in the registry.

Root cause: difflib.get_close_matches scores pred_int vs capability:pred_int below the 0.6 cutoff because the capability: prefix makes them too dissimilar.

Before:

list_estimators_tool(tags={"pred_int": True})
# {'success': False, 'suggestions': {'pred_int': None}}

After:

list_estimators_tool(tags={"pred_int": True})
# {'success': False, 'suggestions': {'pred_int': ['capability:pred_int', 'capability:pred_int:insample']}}

Fix: adds a _suggest_tag helper that first tries difflib, then falls back to substring matching when difflib finds nothing.

Also fixes the docstring which listed only 6 of 9 valid task types (missing parameter_estimation, splitting, network).

Does your contribution introduce a new dependency? If yes, which one?

No. Uses only difflib from the Python standard library, which was already imported.

What should a reviewer concentrate their feedback on?

• The _suggest_tag helper in src/sktime_mcp/tools/list_estimators.py and whether the substring fallback logic is sound
• The 7 new tests in tests/test_list_estimators.py covering invalid task errors, invalid tag errors, substring suggestions, and valid inputs passing cleanly

Any other comments?

Discovered while doing early adopter testing as part of #35.

PR checklist

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• I've added unit tests and made sure they pass locally.