feat: Add GetOrganizationUsageSummary and GetUserUsageSummary to BillingService

[devaniketh]

What this PR does

Implements the enhanced billing platform usage summary endpoints for organizations and users:

• GET /organizations/{org}/settings/billing/usage/summary via BillingService.GetOrganizationUsageSummary
• GET /users/{username}/settings/billing/usage/summary via BillingService.GetUserUsageSummary

These endpoints provide the organization- and user-level counterparts to EnterpriseService.GetUsageSummary.

Checklist

• Unit tests added covering both methods and error paths
• Generated accessor methods and tests via ./script/generate.sh
• ./script/generate.sh --check passed
• ./bin/custom-gcl run ./github/... passed with 0 issues
• ./script/test.sh passed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.53%. Comparing base (3a439fa) to head (2ff0c3e).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4521   +/-   ##
=======================================
  Coverage   98.53%   98.53%           
=======================================
  Files         195      195           
  Lines       17848    17872   +24     
=======================================
+ Hits        17587    17611   +24     
  Misses        261      261           

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[gmlewis]

Thank you, @devaniketh!
LGTM.
Awaiting second LGTM+Approval from any other contributor to this repo before merging.

cc: @stevehipwell - @alexandear - @Not-Dhananjay-Mishra