Accept the subscribed filter as a string in IssuableFinder

What does this MR do and why?

CSV exports that filter by subscription status ignored the filter. GraphQL passes subscribed as a Symbol, but export workers receive their params as JSON, so the value arrives as a String and IssuableFinder#by_subscribed fell through to returning everything. It now compares the value as a Symbol either way.

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)	Extract the merge request filter arguments into a shared concern (no schema change)	None
1	!262242 (merged) (this MR)	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

• Affects any CSV export that passes subscribed through IssuableExportCsvWorker to an IssuableFinder-based finder, including workItemsCsvExport today and the upcoming mergeRequestsCsvExport (!261924 (merged)).
• Also fixes ?subscribed= on HTML issuable lists: subscribed is an IssuableFinder scalar param that IssuableCollections permits from the query string, where it is always a String, so those lists accepted the filter and ignored it.
• New MergeRequestsFinder example passes subscribed: 'explicitly_subscribed' and fails without the fix (it returns all three authored MRs instead of the subscribed one).
• spec/finders/merge_requests_finder_spec.rb and spec/finders/issues_finder_spec.rb: 413 examples, 0 failures.

References

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

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