Fix QKMS async CreateKey and timeout behavior

[thecompez]

What this fixes

• Adds signed CreateKey?async=1 support to match the official Quilibrium QKMS SDK/sample and avoid long-lived MPC/DKG requests being killed by proxy/CDN timeouts.
• Uses QKMS-compatible Content-Type: application/json.
• Marks both blocking and async CreateKey calls non-idempotent so timeout/5xx handling cannot automatically retry and create duplicate key tasks.
• Documents the boundary between this SDK's direct QKMS HTTP surface and participant-side QNZM/MPC sidecar ceremony orchestration.
• Adds facade/service tests for async target, content type, and non-idempotent CreateKey behavior.
• Fixes the stale C API version assertion (1.1.0 -> 1.2.0) exposed by the full test run.

Verification

Clean GNU 16.2 / C++23 / Ninja build with warnings-as-errors completed successfully.

ctest --test-dir build-qkms-clean --output-on-failure:

• 7/7 tests passed
• quilibrium_facade_tests passed
• quilibrium_service_tests passed
• quilibrium_c_api_tests passed
• git diff --check clean

Scope note

This fixes the SDK-side HTTP/timeout behavior. It intentionally does not reimplement the browser/server participant-side MPC ceremony; that layer belongs to Quilibrium's official qkms-sdk. If an async CreateKey task is accepted but the service participant still fails to advance/complete its ceremony task, that is an upstream QKMS/server-side issue rather than this SDK's HTTP wrapper.