minor: add async agent lifecycle support

[manavanandani]

Summary

• await async @on_step and @on_reset hooks on one persistent SDK-managed event loop
• preserve synchronous behavior across stdio and Streamable HTTP
• add explicit --timeout controls with validated ECP_RPC_TIMEOUT fallback
• add contextual RPC failure diagnostics
• provide a runnable async Python example
• cover async success, reset, exceptions, timeouts, stdio, HTTP, and conformance flows

Why

Most agent frameworks are async-first. ECP needs to await lifecycle hooks safely while retaining its synchronous server API and full compatibility with existing agents. Explicit timeouts prevent stalled agents from hanging local and CI evaluation runs.

Validation

• compile verification passed
• SDK suite passed
• runtime suite passed
• flagship customer-support evaluation passed 8/8 checks
• stdio and Streamable HTTP async integrations passed
• GitHub Actions passed across Python 3.9 through 3.13

Migration

No migration is required. Existing synchronous hooks and ECP_RPC_TIMEOUT behavior remain supported.