Fix GitHub App failures when connecting to GitHub Enterprise

[shahzadhaider1]

Fixes GitHub App authentication failures when connecting to GitHub Enterprise Server instances by ensuring the API endpoint includes the required /api/v3 path component.

Problem

Customers using GitHub Apps with GitHub Enterprise Server were encountering 406 Not Acceptable errors during token refresh:

Root Cause

GitHub Enterprise Server requires all API endpoints to include /api/v3 in the path (docs). The issue occurred because:

1. The go-github client library automatically adds /api/v3 via WithEnterpriseURLs() for REST API calls
2. The ghinstallation library uses a separate BaseURL field to construct token refresh URLs as {BaseURL}/app/installations/{id}/access_tokens
3. When users provided endpoint: https://github.company.com (without /api/v3), the ghinstallation library constructed the incorrect URL. Reference: GitHub Enterprise Endpoint

Solution

Added normalizeGitHubEnterpriseEndpoint() function in pkg/sources/github/connector.go that:

• Ensures GitHub Enterprise Server endpoints include /api/v3 before passing to connector implementations
• Leaves GitHub.com endpoints (api.github.com) unchanged

Impact Analysis:

• GitHub App connector: Fixed (needed for ghinstallation)
• Token connector: Safe (WithEnterpriseURLs checks before adding)
• BasicAuth connector: Safe (WithEnterpriseURLs checks before adding)
• Unauthenticated connector: Safe (WithEnterpriseURLs checks before adding)
• GraphQL client: Safe (strips /api/v3 before adding /api/graphql)

Testing

Unit Tests

• Added comprehensive unit tests for normalizeGitHubEnterpriseEndpoint()
• Covers GitHub.com, GitHub Enterprise and edge cases

Manual Testing (GitHub.com)

Verified all authentication methods work correctly with GitHub.com:

• GitHub App: Successfully scanned private and public repos
• Token: Successfully scanned private and public repos
• Basic Auth (username + token): Successfully scanned private and public repos
• Unauthenticated: Successfully scanned public repos

Expected Behavior for GitHub Enterprise Server

With this fix, users can now configure GitHub Enterprise endpoints in any of these formats:

endpoint: https://github.company.com          # Will be normalized to /api/v3
endpoint: https://github.company.com/api/v3   # Already correct, unchanged

Both formats will now work correctly for all authentication types, including GitHub Apps.

Checklist:

• Tests passing (make test-community)?
• Lint passing (make lint this requires golangci-lint)?