[core][python] Add global index fast search option

[JingsongLi]

Summary

Add global-index.fast-search as the freshness/performance switch for global-index queries. It defaults to true for index-only fast search; when set to false, supported global-index reads scan unindexed row ranges and merge fresh raw-data matches.

Changes

• Add the Java and Python global-index.fast-search option with generated docs.
• Use the option in vector search, Spark vector reads, and generic data-evolution global-index scans.
• Remove the unpublished vector raw-fallback option surface and derive vector slow-search metric from the vector indexer interface.
• Add exact unindexed-range scanning for scalar global-index reads so slow search returns only rows matching the predicate.
• Add Java and Python coverage for fast-search defaults, slow-search freshness, filtered unindexed scans, and Python extra-field scalar index matching.

Testing

• git diff --check
• rg -n "raw-fallback|RAW_FALLBACK|raw fallback|raw_fallback|rawFallback|RawFallback" . || true
• python -m compileall -q paimon-python/pypaimon/globalindex/global_index_scanner.py paimon-python/pypaimon/read/scanner/file_scanner.py paimon-python/pypaimon/table/source/vector_search_read.py paimon-python/pypaimon/table/source/vector_search_scan.py paimon-python/pypaimon/common/options/core_options.py paimon-python/pypaimon/tests/vector_search_filter_test.py
• python -m pytest paimon-python/pypaimon/tests/read_builder_explain_test.py paimon-python/pypaimon/tests/vector_search_filter_test.py paimon-python/pypaimon/tests/global_index_test.py -q
• mvn -pl paimon-core -am -Pfast-build -DfailIfNoTests=false -Dtest=VectorSearchBuilderTest#testVectorSearchScansUnindexedDataWhenFastSearchDisabled+testVectorSearchFastSearchSkipsUnindexedDataByDefault+testVectorSearchSlowSearchScansFilteredUnindexedData,BtreeGlobalIndexTableTest#testBTreeGlobalIndexFastSearchControlsUnindexedData test
• mvn -pl paimon-spark/paimon-spark-common -am -Pfast-build -DskipTests compile

Notes

vector.raw-fallback.enabled is intentionally not kept as a compatibility alias because it was not released.