Close bodyless h2c request streams

[OskarEichler]

Summary

• set END_STREAM on the synthesized request HEADERS for a bodyless h2c upgrade
• preserve END_HEADERS, which is already present on the frame
• leave request bodies on the existing DATA + END_STREAM path

Reproduction

Server#upgrade initializes the frame with END_HEADERS, then ORs END_HEADERS a second time when the HTTP/1.1 request body is empty. Stream 1 remains open, although RFC 9113's h2c upgrade semantics require the completed HTTP/1.1 request to be half-closed from the client toward the server.

Verification

• 448 examples, 0 failures on Ruby 4.0.6
• focused bodyless upgrade finishes in half_closed_remote
• body-bearing upgrade behavior remains covered by the existing suite
• syntax and RuboCop pass

No repository tests were changed. I used an AI coding assistant during investigation and verification.