Accept username in BudgetCapUserOverrideInput

What does this MR do and why?

Backend for the bulk CSV import of per-user credit cap overrides. Admins paste username, cap, enabled lines, so the upsertUserBudgetCapOverrides mutation needs to accept usernames, not only global IDs.

• BudgetCapUserOverrideInput: userId becomes optional and a new username argument is added. Exactly one of the two must be set on each override (validates exactly_one_of). Blank usernames are rejected.
• UpsertUserOverrides mutation: usernames are resolved to user IDs with a single case-insensitive User.by_username lookup before forwarding to CustomersDot. If any username cannot be resolved, the whole batch is rejected with Users not found: <usernames>, mirroring the existing behavior for unknown user IDs. Namespace membership validation applies to username-resolved users as well.
• username is marked experiment: { milestone: '19.5' }. The UI that uses it is gated behind the credit_caps_bulk_update feature flag introduced in !257651 (merged), so the backend does not check the flag itself.
• GraphQL docs and introspection schemas regenerated.

No CustomersDot changes are needed. Email support is deferred to a follow-up, per the parent issue.

Implementation status

Part of https://gitlab.com/gitlab-org/gitlab/-/work_items/630981

Piece	Work item	MR	Status
Feature flag credit_caps_bulk_update + scaffold button	https://gitlab.com/gitlab-org/gitlab/-/work_items/630982	!257651 (merged)	✅ Merged
BE: username on BudgetCapUserOverrideInput	https://gitlab.com/gitlab-org/gitlab/-/work_items/630983	!258407 (merged)	⬅️ Current
FE: bulk update overrides modal	https://gitlab.com/gitlab-org/gitlab/-/work_items/630982	!257948 (merged)
FE: CSV parser	https://gitlab.com/gitlab-org/gitlab/-/work_items/630984	!258244 (merged)
FE: integrate modal with BE	https://gitlab.com/gitlab-org/gitlab/-/work_items/630985	!258465 (merged)
FE: CSV parser improvements (follow-up)	https://gitlab.com/gitlab-org/gitlab/-/work_items/631895	!259381 (merged)
Documentation update	https://gitlab.com/gitlab-org/gitlab/-/work_items/632125	!259654 (merged)
Feature flag rollout	https://gitlab.com/gitlab-org/gitlab/-/work_items/631018

References

• Task: https://gitlab.com/gitlab-org/gitlab/-/work_items/630983
• Parent issue: https://gitlab.com/gitlab-org/gitlab/-/work_items/630981
• Epic: https://gitlab.com/groups/gitlab-org/-/work_items/23579
• Feature flag scaffold: !257651 (merged)
• PoC: !257323 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Make sure you have a top-level group where you are an owner and a member user in it, or use an admin in admin mode for instance scope.

2. Open GraphiQL at http://localhost:3000/-/graphql-explorer and run (adjust namespacePath and username):

   mutation {
     upsertUserBudgetCapOverrides(input: {
       namespacePath: "your-group"
       overrides: [
         { username: "some-member", cap: 100, enabled: true }
       ]
     }) {
       userOverrides { user { id username } cap capEnabled }
       errors
     }
   }

3. Expected: the override is resolved to the user and forwarded to CustomersDot (locally this will most likely surface a CDot connection error in errors, which is expected without a CDot setup; the important part is that no Users not found error is raised).

4. Try an unknown username: you get Users not found: <username> and CDot is not called.

5. Try an override with both userId and username, or with neither: you get One and only one of [userId, username] arguments is required.

MR acceptance checklist

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