[ASYNC-READY-T7] Add Windows coverage for native async REST server paths

[keaz]

Outcome

Make native async REST server coverage production-ready on Windows once the async HTTP server path is fully native.

Background

The support matrix still marks REST/async runtime on Windows as partial. Client/runtime async coverage exists, but REST/server-oriented async execution coverage remains primarily non-Windows.

Definition of Done

• Add Windows-capable validation for native async HTTP server request/response paths once std.http_server.async_* is end-to-end native
• Add CI smoke coverage or deterministic gated validation for async accept/read/write/serve on Windows
• Verify typed timeout/connection-closed/malformed-request behavior remains stable on Windows-supported paths
• Update docs to reflect actual Windows support scope
• Run targeted validation plus make ci

Acceptance Criteria

• Windows support claims are backed by real validation for native async REST server paths
• Docs describe the Windows async REST server scope precisely
• No placeholder platform claim remains in the async support matrix

Test Plan

• Windows-focused smoke/integration validation for async HTTP server paths
• Contract/doc tests for support-matrix consistency
• make ci

[keaz]

Started investigation from the current macOS workspace.

Confirmed current Windows-specific gaps:

• Windows CI smoke in .github/workflows/ci.yml covers proc/net/concurrency runtime paths, but not native async HTTP server execution.
• The native async HTTP server execution tests in tests/execution_tests.rs are still gated with #[cfg(not(target_os = "windows"))] (exec_http_server_async_fragmented_body_is_parsed_across_recv_boundaries, exec_http_server_async_api_example_smoke, exec_http_server_async_multi_connection_roundtrip).
• Docs still correctly describe Windows async REST support as partial in docs/ai-agent-rest-guide.md and docs/async-event-loop.md.

No placeholder status change here: the issue remains open until Windows-backed async HTTP server validation is actually added and verified.