feat(search): add COLLECT reducer to FT.AGGREGATE

[nkaradzhov]

Support the experimental COLLECT reducer (Redis Search 8.8), which fetches whole documents or projected fields within each GROUPBY group, with optional DISTINCT, in-group SORTBY, and LIMIT.

• FIELDS '*' emits a bare wildcard token; an explicit list is count-prefixed
• nargs is computed dynamically via pushVariadicWithLength (unlike the fixed-narg reducers), covering FIELDS/DISTINCT/SORTBY/LIMIT
• reuses pushSortByProperty for the nested SORTBY clause
• HYBRID picks up COLLECT automatically (shared parseGroupByReducer)

Description

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Checklist

• Does npm test pass with this change (including linting)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?

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Note

Low Risk
Additive, opt-in aggregate reducer parsing and tests; no changes to auth, data paths, or existing reducer behavior beyond shared parseGroupByReducer.

Overview
Adds client support for the experimental COLLECT GROUPBY reducer on FT.AGGREGATE, so callers can gather full documents or selected fields per group with optional DISTINCT, in-group SORTBY, LIMIT, and AS aliases.

parseGroupByReducer now emits the Redis wire format for COLLECT: FIELDS * vs count-prefixed field lists, optional clauses, and a dynamically computed nargs via pushVariadicWithLength (reusing pushSortByProperty for nested sort). Types extend GroupByReducers with a CollectReducer shape.

Coverage includes unit tests for argument transformation (including real-world-style examples) and an integration test against a new OPEN_UNSTABLE test server (--search-enable-unstable-features yes) that asserts grouped results expose the COLLECT alias as an array of per-document maps.

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