fix: export ToolContext, MCPToolChoice, and RequestUsage from agents

[jawwad-ali]

Summary

ToolContext, MCPToolChoice, and RequestUsage are user-facing types that currently require submodule imports. ToolContext is the documented context type for function tools (docs/context.md — even the repo's own examples import it from agents.tool_context), MCPToolChoice is the ModelSettings.tool_choice value for forcing a specific hosted MCP tool, and RequestUsage is the element type of Usage.request_usage_entries. This re-exports them from the top-level agents package, following the same pattern as #3489 and #3490. Existing submodule import paths keep working, so no docs changes are needed.

Test plan

• Added tests/test_public_type_exports.py: test_tool_context_is_exported_at_top_level, test_mcp_tool_choice_is_exported_at_top_level, test_request_usage_is_exported_at_top_level, and test_exported_names_resolve_to_attributes (asserts every name in agents.__all__ resolves via getattr).
• make format, make lint, and pyright pass; mypy reports only pre-existing errors also present on clean main.
• Full test suite run locally; the new tests pass and the only failures are pre-existing on clean main (local environment-related, unrelated to this change).

Issue number

None — follow-up to the export gaps addressed in #3489/#3490.

Checks

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

[seratch]

Thanks for the suggestion, but we don't intend to export everything from the top level module.