feat: SEARCH, VECTOR_SEARCH and HYBRID_SEARCH table functions

[jackye1995]

Adds Spark SQL table functions for Lance search:

• Registers VECTOR_SEARCH, SEARCH, and HYBRID_SEARCH through the Spark SQL extension.
• Executes vector and full-text search through Lance namespace queryTable; hybrid search fuses namespace query results in Spark.
• Removes the legacy nearest read-option scan path and fails fast with a VECTOR_SEARCH migration message.
• Adds search docs, focused Spark tests, Docker integration coverage, and a targeted GitHub Actions workflow for directory namespace search tests.

Validated with focused Spark tests, Spark 4 compile, Spotless, Docker search CI, full Spark CI, and git diff --check.

[github-actions]

ACTION NEEDED
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