feat: Add enterprise app installation lookup

[danyalahmed1995]

Summary

Adds support for looking up a GitHub App installation for an enterprise.

This implements the new AppsService.FindEnterpriseInstallation helper for:

GET /enterprises/{enterprise}/installation

The method follows the existing app installation lookup pattern already used for organization, repository, and user installations.

Details

• Added FindEnterpriseInstallation(ctx, enterprise) to AppsService
• Uses the enterprise installation endpoint:
  • GET /enterprises/{enterprise}/installation
• Reuses the existing Installation model
• Keeps the implementation aligned with the existing installation lookup helpers
• Adds coverage for:
  • successful enterprise installation lookup
  • bad options handling
  • request/do failure handling

Testing

gofmt -w github/apps.go github/apps_test.go
go test ./github -run 'Apps|Installation'
go test ./...

All tests passed locally.

Fixes #4212

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.45%. Comparing base (b0245f1) to head (43b1593).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4230   +/-   ##
=======================================
  Coverage   97.45%   97.45%           
=======================================
  Files         189      189           
  Lines       19152    19154    +2     
=======================================
+ Hits        18665    18667    +2     
  Misses        270      270           
  Partials      217      217           

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[gmlewis]

The GitHub API docs call this "Get an enterprise installation for the authenticated app" so instead of "FindEnterpriseInstallation", can we use "GetEnterpriseInstallation"?

Wow... I see all the other ones in this file are called "Find*"... I'm not sure how I let that one slide by in code reviews. 😞

OK, let's take this to a vote from our dedicated team of reviewers... which do you like best?

• Option A: Just name this one endpoint "GetEnterpriseInstallation"
• Option B: Leave this one endpoint "FindEnterpriseInstallation"
• Option C: Rename (breaking API change) ALL methods whose documentation URL starts with "get-" to "Get*" instead of "Find*" in this file
• Option D: Something else?

cc: @stevehipwell - @alexandear - @zyfy29 - @Not-Dhananjay-Mishra - @munlicode

[Not-Dhananjay-Mishra]

I vote for Option C.
Because in my opinion, method needs to be consistent with GitHub API docs.