HTTP/2: merge upstream Priority header for proxied requests

HTTP/2: merge upstream Priority header for proxied requests

When nginx proxies a request over HTTP/2 and the upstream returns a
Priority response header, record it as the stream's server priority and
recompute the effective priority by merging it with the client's value
(RFC 9218, Section 8): a parameter present in the response overrides the
client's, a parameter absent leaves it unchanged.  The client and server
inputs are kept separate so a later client PRIORITY_UPDATE does not
discard the server hint.

The merge affects only how this hop schedules the stream; the Priority
header forwarded to the client is left unchanged, so
$upstream_http_priority continues to report what the origin sent.

Assisted-by: opencode:claude-opus-4.8