Changed: Document and reorganize provider bridge helper functions

Improve code readability by adding comprehensive documentation and
renaming helper functions to be more self-explanatory.

Changes:
- Renamed is_same_url -> urls_equal_ignoring_slash
- Renamed validate_fixed_api_url -> validate_fixed_endpoint
- Renamed is_resource_name_env_var -> is_azure_resource_name_env_var
- Renamed is_account_id_env_var -> is_chatgpt_oauth_account_id_env_var
- Renamed is_project_id_env_var -> is_antigravity_project_id_env_var
- Added doc comments to all helper functions explaining purpose and usage
- Reorganized functions into provider-specific sections with dividers
- Moved provider-specific helpers adjacent to their build_* functions

Benefits:
- Function names clearly indicate which provider they relate to
- Documentation explains "why" not just "what"
- Code is easier to navigate with logical grouping

Author: Sewer56