Add explicit zero-body POST transport

[nicodes]

Summary

• Add public HttpClientModule.post_zero_body(endpoint, callback, headers) so a POST can send exactly zero body bytes. post_json({}) still serializes and sends {}.
• Native empty POST sets Content-Length: 0 only on the Godot HTTPRequest path. Web POST omits fetch body and does not set Content-Length, which browsers forbid. Authorization stays a request header. Timeout and error callbacks are unchanged.
• Web fetch uses redirect: 'manual'. A conforming browser exposes an opaque redirect, so the bridge does not follow it. A visible cross-origin Location is also rejected. Native GET still follows redirects through Godot HTTPRequest and can resend Authorization, including to another host; this change does not alter that pre-existing behavior.
• Addon version is 0.0.4.

Tests

• Native TCP capture asserts zero body bytes and exactly one Content-Length: 0. post_json({}) remains 2 bytes.
• Real browser export asserts bearer only in Authorization, zero POST bytes, {} for post_json({}), callback success, and no sink hit for a cross-origin redirect.