fix: allow AgentIdentity through Apps MCP gates

[efrazer-oai]

Summary

PR #18811 routes Codex backend auth through AuthProvider, and PR #18904 adds AgentIdentity JWT login/env support on top. The Apps MCP refresh path still had a few enablement gates that only accepted token-backed ChatGPT auth.

This PR changes those gates to use CodexAuth::uses_codex_backend, matching the rest of the auth-provider migration. AgentIdentity can now reach the Apps MCP refresh path and plugin Apps follow-up flow instead of returning an empty connector list before the provider-backed auth is used.

Stack

1. Route Codex auth through AuthProvider
2. Load AgentIdentity from JWT login/env
3. This PR: allow AgentIdentity through Apps MCP gates

Call sites

Call site	Change
core/src/connectors.rs::list_cached_accessible_connectors_from_mcp_tools	Uses uses_codex_backend before reading cached Apps MCP connectors.
core/src/connectors.rs::list_accessible_connectors_from_mcp_tools_with_environment_manager	Uses uses_codex_backend before constructing the Apps MCP manager with provider-backed auth.
app-server/src/codex_message_processor/plugins.rs	Uses uses_codex_backend when deciding whether newly installed plugin Apps need auth follow-up.

Tests

Tests: focused Rust formatting, scoped cargo check, and scoped clippy fix pass for touched packages.