[codex] Add connector package prototype

[dannyzhang-oai]

Summary

Adds an initial Agents SDK connector package prototype that composes existing SDK primitives instead of introducing a separate runtime.

• Adds Connector, ConnectorComponents, ConnectorPlugin, ConnectorRegistry, connector policy labels, and HostedConnectorAuthorization.
• Allows Agent(connectors=[...]) to include connector-provided direct tools and MCP servers in the existing tool resolution path.
• Adds package loading for shared Codex plugin-style bundles via .codex-plugin/plugin.json, .mcp.json, and optional .app.json hosted connector IDs.
• Adds ConnectorRegistry.from_plugin_records(...) and Connector.from_installed_plugin(...) as a narrow adapter for already-fetched Unified Plugins-style records.
• Hardens installed plugin records with mounted package paths under package_root, auth aliases/refs, app-level server labels, allowed tools, approval/deferred-loading options, and policy label metadata.
• Keeps marketplace install, cloud sync, workspace sharing, admin policy, and package discovery outside the SDK runtime for now.
• Updates examples/connectors/package_demo.py to verify the registry path end to end without an API key, including auth alias resolution, mounted package paths, policy labels, and hosted tool options.
• Preserves existing positional Agent(...) constructor compatibility by appending the new connectors field.
• Adds English docs/API reference coverage for connectors and installed plugin registries.

Test plan

• uv run --frozen pytest tests/test_connectors.py tests/test_connector_demo.py tests/test_source_compat_constructors.py::test_agent_connectors_append_preserves_reset_tool_choice_position -q
• uv run --frozen python examples/connectors/package_demo.py --verify
• uv run --frozen ruff format --check src/agents/connectors.py src/agents/agent.py src/agents/__init__.py examples/connectors/package_demo.py tests/test_connectors.py tests/test_connector_demo.py tests/test_source_compat_constructors.py
• uv run --frozen ruff check src/agents/connectors.py src/agents/agent.py src/agents/__init__.py examples/connectors/package_demo.py tests/test_connectors.py tests/test_connector_demo.py tests/test_source_compat_constructors.py
• uv run --frozen mypy src/agents/connectors.py src/agents/agent.py tests/test_connectors.py tests/test_connector_demo.py tests/test_source_compat_constructors.py
• uv run --frozen pyright src/agents/connectors.py src/agents/agent.py tests/test_connectors.py tests/test_connector_demo.py tests/test_source_compat_constructors.py
• uv run --frozen mkdocs build

mkdocs build still emits the repo's existing localized-doc/autoref warnings, but exits 0. Full make tests, make lint, and make format were not run; this is focused prototype validation.

Issue number

N/A

Checks

• I've added new tests
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass