Fix flaky vertx http2NoConnectionLeak test by using h2c prior knowledge - #3385

feign.vertx.ConnectionsLeakTests.http2NoConnectionLeak has been flaky on CI (e.g. failed on #3384), asserting Expected size: 1 but was: 2 open server connections.

Root cause

The test fires 100 concurrent requests over an HTTP/2 pool capped at one connection (PoolOptions.setHttp2MaxSize(1)) and asserts the server saw exactly one connection.

By default vertx negotiates cleartext HTTP/2 via an HTTP/1.1 Upgrade (HttpClientOptions.DEFAULT_HTTP2_CLEAR_TEXT_UPGRADE = true). During that handshake the connection is still HTTP/1.1, so the pool applies http1MaxSize (default 5) instead of http2MaxSize (1). Under CI load the upgrade window is wide enough that a second connection is opened before the first finishes upgrading to HTTP/2, so the server records 2 connections. (DEFAULT_MAX_CONCURRENT_STREAMS is effectively unlimited, so stream multiplexing is not the issue.) It surfaced after the recent vertx 5.1.0 bump because the full pr-build suite only runs on PRs.

Fix

Connect with h2c prior knowledge (setHttp2ClearTextUpgrade(false)): the connection is HTTP/2 from the first byte, so http2MaxSize(1) deterministically caps it at one connection and the 100 requests multiplex over it. Applied to both the feign-vertx test (run against vertx 5.x via feign-vertx5-test) and the feign-vertx4-test copy.

Validation

ConnectionsLeakTests (both HTTP/1.1 and HTTP/2) passes locally under vertx 5.1.0 (5/5 runs) and vertx 4.5.27.