Add configurable client-level request timeout

[Samuel19012002]

Summary

• add an optional client-level timeout parameter
• keep the existing DEFAULT_TIMEOUT behavior when the parameter is omitted
• document how to configure the timeout in the README

Motivation

All requests currently use a fixed 10-second timeout. Allowing callers to configure it makes the client more flexible for long-running services, including MCP servers, without requiring them to mutate a class constant.

Backward compatibility

Existing Client(api_key) and Client(api_key, proxies) calls continue to use the 10-second default. Subclass and runtime overrides of DEFAULT_TIMEOUT are also preserved when no explicit timeout is provided.

Validation

• verified the existing 10-second default
• verified that a custom timeout is passed to the HTTP session
• verified subclass and runtime DEFAULT_TIMEOUT overrides
• ran git diff --check