DVC Fetch: Silently Ignores Failed Git Authentication

Bug Report

Description

When running fetch or an imported target, DVC tries to fetch the target's repo via git. If git is supplied with incorrect PAT token, DVC does not report authentication failure, instead it appears to fetch and reports "Everything is up to date."

Reproduce

1. Do a dvc import from another repo
2. Run dvc fetch
3. When prompted to authenticate, enter bad PAT token
4. No error is raised!

Expected

DVC should report an auth failure

Environment information

Output of dvc doctor:

$ dvc doctor
DVC version: 3.65.0 (pip)
-------------------------
Platform: Python 3.12.12 on Linux-6.8.0-1044-azure-x86_64-with-glibc2.35
Subprojects:
        dvc_data = 3.17.0
        dvc_objects = 5.2.0
        dvc_render = 1.0.2
        dvc_task = 0.40.2
        scmrepo = 3.6.1
Supports:
        azure (adlfs = 2025.8.0, knack = 0.12.0, azure-identity = 1.25.1),
        http (aiohttp = 3.12.9, aiohttp-retry = 2.9.1),
        https (aiohttp = 3.12.9, aiohttp-retry = 2.9.1),
        ssh (sshfs = 2025.2.0)
Config:
        Global: /home/azureuser/.config/dvc
        System: /etc/xdg/dvc
Cache types: symlink
Cache directory: ext4 on /dev/sdb1
Caches: local
Remotes: azure
Workspace directory: ext4 on /dev/sda1
Repo: dvc, git
Repo.site_cache_dir: /var/tmp/dvc/repo/0d9f8cd50b2b0f4097fcfa7092e688b0

Additional Information (if any):

Start by reproducing the failure through dvc import followed by dvc fetch with an invalid PAT, then trace the Git authentication path used by that entry point. Done means the same failed authentication produces an explicit auth error instead of reporting that everything is up to date.