feat: Add GitHub Enterprise App installation repository management APIs

[nithish-95]

Description

Implements enterprise GitHub App installation repository management endpoints for issue #3829.

This PR adds the following endpoints:

• GET /enterprises/{enterprise}/apps/organizations/{org}/installations/{installation_id}/repositories
• PATCH /enterprises/{enterprise}/apps/organizations/{org}/installations/{installation_id}/repositories
• PATCH /enterprises/{enterprise}/apps/organizations/{org}/installations/{installation_id}/repositories/add
• PATCH /enterprises/{enterprise}/apps/organizations/{org}/installations/{installation_id}/repositories/remove

Changes

• Added ListRepositoriesForOrgInstallation method to list repositories accessible to an app installation
• Added ToggleInstallationRepositories method to update repository selection
• Added AddRepositoriesToInstallation method to add repositories to an installation
• Added RemoveRepositoriesFromInstallation method to remove repositories from an installation
• Added corresponding test cases for all methods
• Added necessary request/response types

Testing

• All unit tests pass
• Linter checks pass
• Follows existing code patterns in the repository

cc @gmlewis @Not-Dhananjay-Mishra

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.44%. Comparing base (903265b) to head (1a69bbf).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3831      +/-   ##
==========================================
+ Coverage   92.41%   92.44%   +0.02%     
==========================================
  Files         197      198       +1     
  Lines       14152    14195      +43     
==========================================
+ Hits        13079    13122      +43     
  Misses        884      884              
  Partials      189      189              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[gmlewis]

Thank you, @nithish-95!
One minor tweak, please, then we should be ready to merge after a second LGTM+Approval from any other contributor to this repo.

cc: @stevehipwell - @alexandear - @zyfy29

[stevehipwell]

How does this PR relate to #3830? They look related to me, which means we likely want to align the implementation including naming. Would it not make sense to wait for #3830 to be merged before reviewing this PR?

[nithish-95]

I've rebased on master to use the existing AccessibleRepository struct and removed the duplicate definition from this PR.
I also updated ListRepositoriesForOrgInstallation to return []*AccessibleRepository and fixed ToggleInstallationRepositories to return *Installation as requested.

I feel sorry for the way I use Git!!

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

[gmlewis]

Thank you, @nithish-95!
LGTM.
Awaiting second LGTM+Approval from any other contributor to this repo before merging.

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