Credit caps overrides bulk update parser improvements

What does this MR do and why?

Addresses review feedback from !258244 (merged) to harden the CSV parser and improve error reporting.

• Pin delimiter to , so Papa Parse doesn't auto-detect and emit UndetectableDelimiter
• Skip blank rows manually in the loop so lineNumber matches the textarea line the user sees
• Require at least one entry — submitting empty input now shows a validation error instead of a parse error
• Have parseOverridesCsvRow return { override } or { error } so cap is computed once

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)	✅ Merged
FE: bulk update overrides modal	https://gitlab.com/gitlab-org/gitlab/-/work_items/630982	!257948 (merged)	✅ Merged
FE: CSV parser	https://gitlab.com/gitlab-org/gitlab/-/work_items/630984	!258244 (merged)	✅ Merged
FE: integrate modal with BE	https://gitlab.com/gitlab-org/gitlab/-/work_items/630985	!258465 (merged)	✅ Merged
FE: CSV parser improvements (follow-up)	https://gitlab.com/gitlab-org/gitlab/-/work_items/631895	!259381 (merged)	⬅️ Current
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

• Addresses part of https://gitlab.com/gitlab-org/gitlab/-/issues/631895
• Part of https://gitlab.com/gitlab-org/gitlab/-/work_items/630981
• Follow-up to !258244 (merged)

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:credit_caps_bulk_update)

2. Visit admin area

3. Then GitLab Credits page

4. Click on Credit caps, leading to http://localhost:3000/admin/gitlab_credits_dashboard/credit_caps

5. Click on Bulk update button

6. Fill csv input and submit

7. See either parsed values or input errors

A few cases for testing:

Empty input — submitting with nothing (or only blank lines) should show "No overrides to save":

Blank lines are ignored — line numbers in errors still match the textarea (line 2 is blank, error should say "Line 3"):

alice,100,true

bob,notanumber,false

Case-insensitive enabled — all of these should parse successfully:

alice,100,True
bob,50,FALSE
carol,0,TRUE

Invalid — enabled value that is not a boolean variant (should error):

alice,100,yes

Semi-colon separated — delimiter is pinned to ,, so this is rejected outright:

alice;100;true
bob;50;false

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.