Add remaining filter tokens to merge request search

What does this MR do and why?

This merge request adds filter tokens for author, reviewer, approved-by, merge user, milestone, label, my reaction, draft, merged before, merged after and subscribed, together with the autocomplete paths they need.

Only tokens that genuinely narrow the result set count towards the query guard, so wildcard values like Any and None, and bare negations, still leave the list unqueried.

References

Split from !252201 (closed)

Part of a stack:

1. !252305 (merged) - feature flag and backend enablement
2. !252306 (merged) - Vue search list
3. !252307 (merged) - (this merge request) remaining filter tokens
4. !252308 (merged) - row details
5. !252309 (merged) - approval status

Screenshots or screen recordings

Before	After

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