feat: Add enterprise Visual Studio subscription licensing APIs

[hariom-hp]

Summary

Adds typed EnterpriseService support for GitHub Enterprise Cloud's Visual Studio Subscriptions (VSS) licensing endpoints:

• ListVisualStudioSubscriptions: GET /enterprises/{enterprise}/visual-studio-subscriptions
• AddOrUpdateVisualStudioSubscriptionUserMatch: PUT /enterprises/{enterprise}/visual-studio-subscriptions/{visual_studio_subscription_id}
• DeleteVisualStudioSubscriptionUserMatch: DELETE /enterprises/{enterprise}/visual-studio-subscriptions/{visual_studio_subscription_id}

Changes

• Defined VisualStudioSubscriptions, VisualStudioSubscriptionAssignment, VisualStudioSubscriptionUserMatchRequest, and ListVisualStudioSubscriptionsOptions.
• Added the three licensing methods on EnterpriseService with OpenAPI //meta:operation directives and API documentation links.
• Added unit tests in enterprise_licenses_test.go covering success cases, request body payloads, HTTP 204 responses, bad options, and mock server failures (100% statement coverage).
• Regenerated accessors in github-accessors.go and iterators in github-iterators.go.

Validation

• script/fmt.sh
• script/generate.sh --check
• script/lint.sh (0 issues across all 12 modules)
• script/test.sh -race -covermode atomic ./... (0 failures across all 12 modules)
• go test -v -tags=integration -run=^$ ./test/integration
• git diff --check

[google-cla]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.52%. Comparing base (30af001) to head (782156e).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4483   +/-   ##
=======================================
  Coverage   98.51%   98.52%           
=======================================
  Files         195      195           
  Lines       17691    17735   +44     
=======================================
+ Hits        17429    17473   +44     
  Misses        262      262           

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[gmlewis]

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

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