Implement Forgejo platform provider using Gitea SDK

[bloboss]

This PR implements the complete Forgejo platform provider by integrating the Gitea SDK, replacing all placeholder "not yet implemented" methods with functional implementations.

Summary

The Forgejo provider now fully implements the platform.Provider interface by leveraging the code.gitea.io/sdk/gitea client library. All provider methods across repositories, users, organizations, teams, collaborators, and branches are now functional.

Key Changes

• Lazy SDK initialization: Added sdk() method that lazily initializes the Gitea client using sync.Once to avoid blocking on network availability during provider construction
• Error translation: Implemented translateError() function that maps Gitea SDK errors and HTTP status codes to platform-specific sentinel errors (ErrNotFound, ErrUnauthorized, ErrForbidden, ErrAlreadyExists, ErrRateLimited, ErrUnavailable)
• Type conversions: Added helper functions to convert between Gitea SDK types and platform types:
  • toRepository(), toUser(), toOrganization(), toTeam()
  • toAccessMode() and toPlatformPermission() for permission mapping
• Repository operations: Implemented GetRepository(), GetRepositoryByID(), CreateRepositoryFromTemplate(), and DeleteRepository()
• User operations: Implemented GetAuthenticatedUser(), GetUser(), and GetUserByEmail() with email matching logic
• Organization operations: Implemented GetOrganization(), CreateOrganization(), and DeleteOrganization()
• Team operations: Implemented CreateTeam(), DeleteTeam(), AddTeamMember(), RemoveTeamMember(), AddTeamRepository(), and RemoveTeamRepository()
• Collaborator operations: Implemented AddCollaborator() and RemoveCollaborator()
• Branch operations: Implemented ProtectBranch() and GetLatestCommit()
• Tests: Replaced placeholder tests with functional tests using httptest to verify error translation behavior

Notable Implementation Details

• The Gitea SDK's NewClient() may return both a usable client and an ErrUnknownVersion error; the implementation tolerates this case
• Team creation includes default repository units (Code, Issues, Pulls) and disables "includes all repositories"
• Repository template creation copies git content, topics, labels, and avatars
• GetUserByEmail() performs case-insensitive email matching since the Gitea search API doesn't guarantee exact matches
• Branch protection uses the branch name as both the branch name and rule name
• Commit retrieval handles cases where commit metadata may be partially populated

https://claude.ai/code/session_01TMo5LrrvEyEc3JfxbvPts6