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 API and command encoding for an optional reducer; existing reducers unchanged, with coverage focused on argument transformation and opt-in unstable server tests.

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 serializes COLLECT with dynamic nargs via pushVariadicWithLength (wildcard FIELDS * vs count-prefixed field lists, plus optional clauses). HYBRID inherits COLLECT through the shared reducer parser. Tests add transform coverage and integration runs against OPEN_UNSTABLE (Redis 8.10+ with unstable search features).

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