refactor: multiple feats and fixes

[usamoi]

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[Copilot]

Pull request overview

This PR tightens bm25 query/index correctness and expands configurability by adding per-index prefilter reloptions (with GUC fallback behavior), while also refactoring bm25 index build code paths and extending sqllogictest coverage.

Changes:

• Add a RelationId trait and wire it through scanner builders so a bm25query can validate it’s used with the intended index.
• Introduce prefilter as a bm25 index reloption and update GUC handling to fall back to reloptions when the GUC is default.
• Refactor bm25 index build implementation to consolidate sequential/parallel build logic; add/extend sqllogictest cases.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/sqllogictest/fallback_parameter.slt	Adds coverage for prefilter reloption affecting filtered queries under a low bm25 limit.
tests/sqllogictest/bm25query.slt	New test ensuring to_bm25query(..., index_name) errors if the planner uses a different (e.g., partial) bm25 index.
src/lib.rs	Adds a main-process assertion for temporary-name generation.
src/index/storage.rs	Implements RelationId for PostgresRelation using the underlying relation OID.
src/index/scanners.rs	Requires RelationId for SearchBuilder::new so builders can capture the index identity.
src/index/gucs.rs	Adds config pointer tracking for bm25.prefilter and implements GUC-vs-reloption precedence for bm25_prefilter.
src/index/bm25/scanners/default.rs	Captures index OID in the builder and validates the bm25query.index matches the scan’s index.
src/index/bm25/am/mod.rs	Adds prefilter to bm25 reloptions parsing/registration and wires bm25_prefilter(relation) into scan options.
src/index/bm25/am/am_build.rs	Refactors parallel/sequential build into a shared build function and factors out DSM path parsing.
crates/index/src/relation.rs	Introduces the RelationId trait in the index crate API.

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