feat: Add GetAICreditUsage endpoint for organization AI credits

[maishivamhoo123]

Closes #4262

Adds the GetAICreditUsage method to BillingService to support the
GET /organizations/{org}/settings/billing/ai_credit/usage endpoint.

Changes:

• Reuses PremiumRequestUsageReportOptions and PremiumRequestUsageReport
  since the request/response shapes are identical
• Adds unit tests covering successful response, float quantities, invalid
  org, and request/do failure
• All code lives in the existing billing.go and billing_test.go files
  per reviewer feedback from feat: Add GetAICreditUsage endpoint for organization AI credits #4264

[maishivamhoo123]

@gmlewis I updated the code in billing.go as Recommended by @alexandear Ready for Review
Thank you!

[gmlewis]

It looks like #4281 and #4282 have overlapping functionality.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.49%. Comparing base (531d1cf) to head (67ed937).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4282   +/-   ##
=======================================
  Coverage   97.49%   97.49%           
=======================================
  Files         192      192           
  Lines       19230    19243   +13     
=======================================
+ Hits        18748    18761   +13     
  Misses        267      267           
  Partials      215      215           

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[maishivamhoo123]

@gmlewis @Not-Dhananjay-Mishra can you please review this?
Thank you!

[gmlewis]

@gmlewis @Not-Dhananjay-Mishra can you please review this? Thank you!

Are you interested in supporting the User endpoint in addition to the Organization endpoint in this PR as demonstrated by #4281, or what are your plans for this?

It seems to me like having a GetOrgAICreditUsage and GetUserAICreditUsage side-by-side would be extremely useful.

Thoughts?

[alexandear]

Suggested change

	func (s *BillingService) GetAICreditUsage(ctx context.Context, org string, opts *PremiumRequestUsageReportOptions) (*PremiumRequestUsageReport, *Response, error) {
	func (s *BillingService) GetOrgAICreditUsage(ctx context.Context, org string, opts *PremiumRequestUsageReportOptions) (*PremiumRequestUsageReport, *Response, error) {