[connectors] Ignore synthetic links for app accessibility

[adaley-openai]

Summary

• Stop treating Codex Apps MCP tools with _meta._codex_apps.synthetic_link: true as evidence that a connector is accessible in app/list.
• Preserve synthetic tools in the agent-facing MCP connector set so they remain available for install/auth flows.
• Keep the app-list accessibility cache limited to connectors backed by at least one non-synthetic tool.
• Add focused regression coverage for both sides of the boundary.

Validation

• just fmt
• just test -p codex-core synthetic_links_are_exposed_to_the_agent_but_not_accessible_in_app_list
• git diff --check
• A crate-wide just test -p codex-core run completed with 2,699 passing and 51 unrelated local sandbox/state failures, primarily state DB migration races (UNIQUE constraint failed: _sqlx_migrations.version).