Add CSV parser for bulk update overrides

What does this MR do and why?

Adds csv parsing to credit caps overrides bulk edit modal

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)	⬅️ Current
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

• Part of https://gitlab.com/gitlab-org/gitlab/-/work_items/630982
• Depends on: !257651 (merged)

Screenshots or screen recordings

Before	After

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. there 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:

Happy path — mixed enabled/disabled, integer and fractional caps:

alice,100,true
bob,50.5,false
carol,0,true

Quoted username with a comma (tests Papa Parse quoting):

"smith,john",200,true
jane,75,false

Invalid — one bad line among good ones (should show parse error, no output):

alice,100,true
bob,notanumber,false
carol,50,true

Invalid — @ in username:

alice@example.com,100,true
bob,50,false

Edge — cap of zero and false enabled:

alice,0,false
bob,999.99,true
carol,0,true
dave,1,false

MR acceptance checklist

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