[core][python] Add global index search modes

[JingsongLi]

Summary

Add global-index.search-mode as the freshness/performance switch for global-index queries. It defaults to fast for index-only reads, supports full to use snapshot nextRowId plus index coverage before scanning raw data, and supports detail to scan data file metadata for exact unindexed ranges caused by updates or rewrites.

Changes

• Replace the unreleased global-index.fast-search option with global-index.search-mode = fast | full | detail in Java, Python, and generated docs.
• Make scalar global-index scans and vector search honor the three search modes.
• In full mode, use snapshot nextRowId and global-index row-id coverage to avoid planning all data files unless an uncovered range exists.
• In detail mode, scan data file metadata to detect exact unindexed row ranges, including invalidation caused by file rewrites or updates.
• Carry snapshot nextRowId through Java and Python vector scan plans so vector raw-data search can use the lightweight full path.
• Update Java and Python tests for default fast mode, full-mode freshness, filtered unindexed scans, and detail-mode exact range detection.

Testing

• git diff --check
• rg -n "fast-search|global-index\\.fast-search|GLOBAL_INDEX_FAST_SEARCH|globalIndexFastSearch|global_index_fast_search|slowSearch|slow search|FastSearch" paimon-api paimon-core paimon-python docs/docs docs/generated
• python -m compileall paimon-python/pypaimon/common/options/core_options.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
• python -m pytest paimon-python/pypaimon/tests/global_index_test.py::PlanSnapshotFetchRegressionTest::test_search_mode_detail_filters_unindexed_rows_exactly paimon-python/pypaimon/tests/vector_search_filter_test.py::VectorSearchManySplitsTest::test_search_mode_controls_unindexed_range_scan
• mvn -pl paimon-api,paimon-core -DskipTests spotless:check
• mvn -pl paimon-core -am -Pfast-build -DskipTests -DfailIfNoTests=false compile test-compile
• mvn -pl paimon-core -am -Pfast-build -DfailIfNoTests=false -Dtest=VectorSearchBuilderTest#testVectorSearchFullModeScansUnindexedData+testVectorSearchFastModeSkipsUnindexedDataByDefault+testVectorSearchFullModeScansFilteredUnindexedData,BtreeGlobalIndexTableTest#testBTreeGlobalIndexSearchModeControlsUnindexedData test

Notes

global-index.fast-search is intentionally not kept as a compatibility alias because it has not been released.