[lszskye]

Purpose

Linked issue: #452
Existing search setters cannot represent full-text conditions inside mixed
AND/OR filters, such as body matches "apple" OR price < 10.

This PR introduces the predicate semantics and interfaces for these queries:

• Add FullTextMatch as a leaf in the existing predicate tree, using the
  same JSON DSL as FullTextSearch without TopK truncation.
• Preserve full-text conditions during projection, field reordering, and
  schema renaming without modifying the original shared request.
• Retain the root TopK setters and validate positive limits and nonempty
  search inputs.
• Resolve explicit _<field>_INDEX_SCORE projections and bind each output
  position to a search type and original table field name.
• Reject missing or ambiguous score sources and score-producing full-text
  conditions beneath OR. Root TopK requires internal scores regardless
  of projection.

This PR covers construction, validation, and binding. Full-text predicate
execution, scan pruning with full-text predicates, and per-field score
materialization return NotImplemented. Mixed-query execution, cross-file
TopK merging, and selectivity-based scheduling will follow separately.

Tests

Added coverage for:

• Predicate construction, equality, AND/OR composition, and unsupported
  scalar/statistics evaluation.
• Projection validation and field rename/reorder mapping.
• Score bindings, projection precedence, duplicate sources, and OR restrictions.
• Root TopK score requirements and search-type isolation.
• Explicit rejection of unsupported scan and read execution.
• Cross-module casts from the new predicate to LeafPredicate.

API and Format

Adds:

• FullTextMatch
• PredicateBuilder::FullTextMatch
• PredicateUtils::ContainsFullTextMatch
• Function::Type::FULL_TEXT_MATCH
• FileIndexReader::VisitFullTextMatch

Documentation

Document predicate composition, root TopK semantics, score projection rules,
and current execution limitations in docs/source/api/file_index.rst.

Generative AI tooling

Generated-by: OpenAI Codex (GPT-6)

[lxy-9602]

+1