Extract OAuth credentials from settings module

[DevonFulcher]

Why

settings.py has grown into a large file mixing configuration (settings classes, validators) with authentication orchestration (OAuth flow, token refresh, credential resolution). This makes the module harder to navigate and creates a tight coupling between unrelated concerns.

What

• Created src/dbt_mcp/config/credentials.py containing all auth orchestration logic:
  • AuthenticationMethod enum
  • CredentialsProvider class (with updated OAuth URL construction via _build_dbt_platform_url and base_host property from the base branch)
  • get_dbt_platform_context, get_dbt_profiles_path
  • Internal helpers: _find_available_port, _is_context_complete, _is_token_valid, _try_refresh_token
  • OAUTH_REDIRECT_STARTING_PORT constant
• Slimmed down settings.py to focus on configuration classes, validators, and host prefix utilities (HostPrefixResult, parse_host_prefix, _build_dbt_platform_url)
• Updated all importers to use dbt_mcp.config.credentials directly (no re-exports)
• Updated test patch targets to point at dbt_mcp.config.credentials where the functions now live
• Used from __future__ import annotations and TYPE_CHECKING in credentials.py to avoid circular imports with DbtMcpSettings

Notes

• Rebased onto the latest base branch which added _build_dbt_platform_url, HostPrefixResult, parse_host_prefix, base_host property, and the dbt_token warning in the OAuth fallback path. Those changes are incorporated into the extraction.
• The old get_dbt_host function (replaced by _build_dbt_platform_url + base_host in the base branch) is no longer present.

Drafted by Claude Opus 4.6 under the direction of @DevonFulcher