fix: render Content-Length for HEAD responses with a declared length

[pjfanning]

cherry pick dd0a4a3 (#1237)

Motivation:
PR #962 corrected Content-Length rendering for 205 and CONNECT, but it also made HEAD responses drop the header unconditionally. RFC 9110 section 8.6 allows a server to send Content-Length in a HEAD response, and RFC 9112 section 6.3 rule 1 exempts HEAD responses from body framing, so the header is metadata there and cannot desync a connection. Suppressing it breaks the common case of using HEAD to learn the size of a resource without fetching it, and it is asymmetric: our own client parser honours a HEAD Content-Length, and the HTTP/2 renderer never consulted contentLengthAllowed at all.

Modification:
Restore the pre-1.4 predicate for HEAD (contentLengthAllowedForHead now defers to StatusCode.allowsEntity) and add the length-based policy at render time: a HEAD response renders Content-Length only when the entity declares a length greater than zero. A zero length nearly always means the application had no body to hand over rather than that the resource is empty, and the client parser already only honours a HEAD Content-Length when it is greater than zero. Chunked and CloseDelimited entities have no known length and are unchanged. 205, 204, 304 and CONNECT behaviour from #962 is untouched.

Result:
A HEAD response completed with HttpEntity.Strict or HttpEntity.Default(contentType, length, Source.empty) renders the declared Content-Length again, while an empty entity renders none.

Tests:

• sbt "http-core / Test / testOnly org.apache.pekko.http.impl.engine.rendering.ResponseRendererSpec org.apache.pekko.http.impl.engine.server.HttpServerSpec org.apache.pekko.http.scaladsl.model.HttpMethodsSpec" - 110 passed
• sbt "http-core / Test / testOnly org.apache.pekko.http.impl.engine.client.HostConnectionPoolSpec" - 66 passed
• sbt +mimaReportBinaryIssues - success
• scalafmt --mode diff-ref=upstream/main - clean
• git diff --check - clean
• sbt "http-core / test" and sbt "docs / paradox" - not run to completion locally, left to CI

References:
Fixes #1236, Refs #962