Affected version

3.9.0+

Bug description

Summary

When using Maven 3.9.x behind an HTTP proxy requiring Basic authentication (Squid), Maven intermittently sends CONNECT requests without Proxy-Authorization when running with default artifact resolution threads.

This results in intermittent 407 Proxy Authentication Required errors.
Limiting resolution to a single thread (-Dmaven.artifact.threads=1) makes the problem disappear.

Title: Maven 3.9.x intermittently sends unauthenticated CONNECT to proxy when using multiple artifact threads (407 from Squid)

Summary:
When using Maven 3.9.x behind an HTTP proxy requiring Basic authentication (Squid), Maven intermittently sends CONNECT requests without Proxy-Authorization when running with default artifact resolution threads.

This results in intermittent 407 Proxy Authentication Required errors. Limiting resolution to a single thread (-Dmaven.artifact.threads=1) makes the problem disappear.

The same setup works reliably with older Maven versions (3.8. and lower / Wagon-based resolver).

Environment:

1. Maven: 3.9.11
2. Java: 17
3. OS: Windows 11 (Docker Desktop, Linux containers)
4. Proxy: Squid 6.x with Basic auth required
5. Transport: Maven Resolver HttpTransporter (default in 3.9.x)

Expected behavior:
All HTTP CONNECT requests sent through the proxy should include valid Proxy-Authorization headers once credentials are configured in settings.xml.
No unauthenticated CONNECT attempts should occur after proxy auth is configured.

Actual behavior:
When resolving artifacts in parallel (default behavior), Maven sometimes:
Sends CONNECT without Proxy-Authorization
Receives 407 Proxy Authentication Required
Fails the build

Squid logs show requests like:
"CONNECT repo.maven.apache.org:443 HTTP/1.1" 407 ProxyAuth:- UA:Apache-Maven/3.9.11

Followed later by authenticated requests that succeed.
This does not occur when limiting resolver threads to 1.

Reproducer:
Full reproducer with Docker + Squid is available here:
https://github.com/Eyjafjallajokull-Roman/maven-proxy-auth-repro

It includes:

1. Squid proxy requiring Basic auth
2. Maven 3.9.11 container
3. settings.xml with proxy auth
4. Exact commands to reproduce

How to reproduce:

Ensure Maven local repository is clean so artifacts are downloaded through the proxy
(no cached artifacts in ~/.m2/repository).
Configure Maven to use an authenticated HTTP proxy (see provided settings.xml and Squid setup).

Run with default resolver threads:
mvn -U -X -s settings.xml clean verify

Observed:
Intermittent 407 Proxy Authentication Required errors.

Run with a single resolver thread:
mvn -U -X -Dmaven.artifact.threads=1 -s settings.xml clean verify

Observed:
Build succeeds and proxy authentication errors disappear.

Possible root cause:
It appears that concurrent artifact resolution in HttpTransporter causes some proxy connections to be established before authentication headers are attached, leading to unauthenticated CONNECT attempts.

Could you please advise:
Whether this is expected behavior with the new transport?
Whether proxy authentication is expected to be preemptive for CONNECT?
And whether this might be a known issue or regression in Maven 3.9.x / Resolver?
I am happy to test patches or provide further diagnostics.

(All logs included in repro repo.)