Load full MR reviewer and assignee set in sidebar

What does this MR do and why?

An MR can persist up to 200 reviewers (and assignees), but the reviewers and assignees connection fields on MergeRequestType inherited the schema-wide default_max_page_size of 100. The sidebar widget loaded only the first 100 and, on any save, overwrote the MR's full reviewer set with what it held in memory, silently removing every reviewer past the first 100 with no warning.

This sets both fields' max_page_size to the backend limit constant (Issuable::MAX_NUMBER_OF_ASSIGNEES_OR_REVIEWERS) so the GraphQL cap matches the persistence limit and cannot drift from it. That drift is what caused the bug: the limit was raised from 100 to 200 in 15.9 but the page-size cap was never updated. Using the constant, rather than a literal, keeps the two in lockstep.

Clients that select these fields without an explicit first: now load the full set through this single cap (the connection resolves its limit as min(first, last, max_page_size), which falls through to max_page_size when no first: is given), so no query needs to hardcode the limit.

This is part 1 of a decomposed fix. Part 1 closes the current data-loss window. A follow-up will switch the sidebar to delta (append/remove) mutations so a single-reviewer edit can never reconcile against a truncated client list, removing the failure mode independent of the page-size cap.

Part	Purpose	MR
1	Cap reviewer/assignee page size at the backend limit	👈🏻 You are here
2	Switch sidebar to delta (append/remove) mutations	follow-up

References

Resolves #606591

Related:

• #547908 (closed) (root-cause model raised June 2025)
• #366485 (closed) (raised the backend limit 100 to 200)
• !99163 (merged) (implemented that increase)

Screenshots or screen recordings

No visible UI change; this changes a GraphQL field's page-size cap.

How to set up and validate locally

1. Create an MR and add more than 100 reviewers (the REST reviewers endpoint permits up to 200).
2. Before this change, the sidebar reviewer widget shows only 100.
3. With this change, open the MR sidebar: it loads the full reviewer set.
4. Add one more reviewer and save; confirm no previously-assigned reviewer is dropped.

Automated coverage:

• spec/graphql/types/merge_request_type_spec.rb asserts both fields' max_page_size equals the backend constant.
• spec/requests/api/graphql/project/merge_request_spec.rb loads an MR with more reviewers than default_max_page_size and asserts they are all returned.