[GH-3344] GeoPandas: expose valid spatial index predicates

[jiayuasu]

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Yes, I have read the Contributor Rules and Contributor Development Guide.

Is this PR related to a ticket?

Yes, the PR name follows the format [GH-XXX] my subject.

Closes #3344. Part of #2230.

What changes were proposed in this PR?

Add the read-only SpatialIndex.valid_query_predicates property. It returns a fresh set containing None, "intersects", and "contains", using the same supported-predicate list as query().

This only exposes the accepted predicate values. It does not add predicates or change query execution.

How was this patch tested?

From the python directory:

python -m pytest tests/geopandas/test_sindex.py -q --disable-warnings

• Spark 3.5.4: 21 passed.
• Spark 4.1.1: 21 passed.
• The four new parameterized cases failed with the missing-property AttributeError before implementation. They cover querying with every advertised predicate on local and Spark-backed indexes, empty indexes, and isolation of the returned set.
• Repository formatting and lint hooks passed for both changed files.

Did this PR include necessary documentation updates?

Yes, this adds a public API. Its docstring documents the returned predicates and the default behavior, with versionadded:: 2.0.0 for the current v2.0.0 development release.