[test/integration] Defer fake upstream read enable until initialize

[nezdolik]

When FakeUpstream hands off HTTP connections via consumeConnection(), reads were re-enabled before the codec/filter stack was initialized. This could cause early request bytes to be consumed without creating a FakeStream. Defer read re-enable until FakeHttpConnection::initialize() attaches the HTTP read filter.

Commit Message: [test/integration] Defer fake upstream read enable until initialize
Additional Description:
Risk Level: Low (test code)
Testing:
Docs Changes: NA
Release Notes:
Platform Specific Features:

[nezdolik]

cc @jrajahalme

[Copilot]

Pull request overview

This PR adjusts the FakeUpstream HTTP handoff flow so that when a connection is moved from new_connections_ to a FakeHttpConnection, network reads aren’t re-enabled until the HTTP codec/filter stack is ready, avoiding a race where early request bytes can be consumed before a FakeStream is created.

Changes:

• Add a FakeHttpConnection::initialize() override and use it as the point to (re-)enable reads when appropriate.
• Extend FakeUpstream::consumeConnection() with defer_read_enable to optionally postpone readDisable(false).
• Update HTTP connection creation paths to pass defer_read_enable=true and rely on initialize() to re-enable reads.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
test/integration/fake_upstream.h	Declares FakeHttpConnection::initialize() override; extends consumeConnection() API with defer_read_enable.
test/integration/fake_upstream.cc	Implements deferred read re-enable in FakeHttpConnection::initialize() and updates HTTP connection creation to defer read enabling.

[kyessenov]

CI failure looks real.
/wait