fix(postgres): flatten multi-value _id predicates

[Gordex2014]

Fixes #1413.

What changed

• Build positive multi-value _id filters as a flat PostgreSQL IN predicate instead of a left-deep OR tree.
• Keep single-value _id and all _id:not behavior unchanged.
• Add regression coverage for 12,000 IDs across result and count queries.
• Assert that forward chain and _has resolution produce positive _id filters.

Compatibility

Tenant and resource type scoping, placeholder offsets, repeated-parameter AND behavior, _list, _include, and Bulk Export matching semantics are unchanged.

Validation

• PostgreSQL test target: 268 passed, 0 failed.
• The 12,000-ID regression passed in PostgreSQL 16 for both search() and search_count().
• Forward chain, _has, wide two-level chain, single _id, and _id:not tests passed.
• Formatting, diff checks, and standard Clippy completed successfully.

Strict Clippy with -D warnings remains blocked by existing Rust 1.97 warnings outside the changed code.

Visual explanation

View the nested OR versus flat IN explanation.

Temporary review evidence expires 2026-09-26T16:55:00.190Z.

Known limit

The query still uses one bind per ID. Lists near PostgreSQL's protocol limit of roughly 65,000 binds require a separate array-binding change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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