DEV: (cmds) FT.AGGREGATE - add COLLECT reducer

[dwdougherty]

Redis 8.8 feature

@Itzikvaknin I couldn't add you directly as a docs reviewer, but I'm hoping you can review as a guest (just leave a comment).

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Note

Low Risk
Documentation-only changes; primary risk is inaccuracies in the new COLLECT syntax/semantics or generated diagram updates confusing users.

Overview
Adds documentation for the new REDUCE COLLECT reducer in FT.AGGREGATE (Redis 8.8.0), including the formal argument/schema definition, detailed reducer reference, and a new end-to-end example for collecting top documents per group.

Expands the FT.AGGREGATE command docs with additional argument summaries and a much more explicit APPLY expression/function grammar, and updates the ft.aggregate railroad diagram (static/images/railroad/ft.aggregate.svg) to reflect the new syntax.

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[github-actions]

DOC-6571

[github-actions]

Staging links:
https://redis.io/docs/staging/DOC-6571/commands/ft.aggregate
https://redis.io/docs/staging/DOC-6571/commands/ft.aggregate/
https://redis.io/docs/staging/DOC-6571/develop/ai/search-and-query/advanced-concepts/aggregations/

[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[andy-stark-redis]

Language LGTM.

[dwdougherty]

TYVM, @andy-stark-redis!

[Itzikvaknin]

I’m not sure whether Fetch is the right term here. Since you’re already familiar with the issue I mentioned in the other comments, I’ll leave it to your judgment.

[Itzikvaknin]

[Itzikvaknin]

Looks good!
I left a few comments to improve clarity and correctness.

[dwdougherty]

Thank you, @Itzikvaknin!

[dwdougherty]

This can only be fixed by modifying the commands.json file, because that's the source of the railroad diagrams.

[oshadmi]

@Itzikvaknin Also need nargs after COLLECT