Extract merge request filter arguments into a concern

What does this MR do and why?

Moves the merge request GraphQL filter arguments out of Resolvers::MergeRequestsResolver into a shared concern, so the upcoming mergeRequestsCsvExport mutation (!261924 (merged)) can accept exactly the same filters as the list. No behavior or schema change.

Merge request stack

These merge requests fix #583992 together. Merge them in this order; same-numbered MRs are independent.

Order	MR	What	Depends on
1	!262241 (merged) (this MR)	Extract the merge request filter arguments into a shared concern (no schema change)	None
1	!262242 (merged)	Accept the subscribed filter as a string in IssuableFinder	None
2	!261924 (merged)	Add the mergeRequestsCsvExport GraphQL mutation	!262241 (merged), !262242 (merged)
3	!261946 (merged)	Switch the MR list export to the mutation behind merge_requests_csv_export_graphql	!261924 (merged)
4	!261956	Remove the REST export and the flag after the rollout in #633656 (not before 2026-11-16)	!261946 (merged)

Details

• MergeRequests::SharedFilterArguments holds the filter arguments, the accept_assignee/accept_author/accept_reviewer helpers, and finder param preparation (blob path and closed range validation, wildcard and assignee rewrites). It mirrors WorkItems::SharedFilterArguments.
• The dynamic negated do block becomes an explicit Types::MergeRequests::NegatedMergeRequestFilterInputType that keeps the GraphQL name MergeRequestsResolverNegatedParams. NegatableArguments names the type after the class it runs in, so a shared concern would otherwise create a second type.
• The EE approver argument moves from EE::Resolvers::MergeRequestsResolver into EE::MergeRequests::SharedFilterArguments, wired in with include_mod.
• Schema: doc/api/graphql/reference/_index.md is unchanged. The introspection JSON diff only reorders arguments (equal additions and deletions).
• Specs: 1280 examples across the MR resolver specs and project, group and EE MR GraphQL request specs pass. The resolver unit spec drops prepare_assignee_username_params, which moved into prepare_finder_params.

References

• Related to #583992
• Split out of !261924 (merged) per review.

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