fix Invocation.Builder.property() not visible to HTTP transport

[neenapj]

Fixes CXF-9235

Made changes to InvocationBuilderImpl.property() to write directly into contextProps, matching how Client/WebTarget already behave.

Tests added (InvocationBuilderImplTest)

testPropertyWrittenToBothFilterPropsAndFlatContext sets SET_EMPTY_REQUEST_CT_PROPERTY = false via Invocation.Builder.property() and asserts two storage locations on ClientConfiguration.getRequestContext():

(a) filterProps sub-map (requestContext.get("jaxrs.filter.properties")): consumed by ClientRequestFilter.getProperty() via MessagePropertyHolder. This path was always written; the test guards it stays working.
(b) flat top-level map (requestContext.get("set.content.type.for.empty.request")): consumed by the HTTP transport via Message.getContextualProperty() in Headers.setProtocolHeadersInConnection(). This was the CXF-9235 bug — only (a) was written, so the transport silently ignored the property. The test fails on (b) when the fix is reverted (expected: but was:), and passes once the flat write is restored. The test also verifies that passing null removes the property from both locations.
No network call is made — the test inspects the ClientConfiguration map directly, since InvocationBuilderImpl.property() populates it synchronously before any dispatch.

[neenapj]

@ffang Could you please take a look at this PR when you get a chance?

[ffang]

Thanks for the contribution @neenapj !