GitHub Enterprise sign-in fails with "No auth flow succeeded" using OAuth and PAT on VPN

🐛 Describe the bug
VS Code fails to authenticate with our GitHub Enterprise server (https://git.cedacrigroup.it) via any supported auth flow.
After attempting both OAuth and Personal Access Token (PAT), the error Sign in failed: Error: No auth flow succeeded is always shown.

🔁 To Reproduce
Steps to reproduce the behavior:
Connected to company VPN
Open VS Code (latest version)

Sign in to GitHub Enterprise:
OAuth → browser opens, but fails to complete flow
Device code → hangs or fails
PAT → accepted, then "No auth flow succeeded" error
VS Code never completes login

✅ Expected behavior
Login to GitHub Enterprise (git.cedacrigroup.it) should complete either via OAuth, device code, or PAT.
The GitHub Pull Requests and Issues extension should be able to authenticate properly.

📷 Screenshots
OAuth failure:
Sign in failed: Error: No auth flow succeeded.

PAT failure:
Same message after entering token.

Extension prompt appears but does not finish.

📦 Extension Version
github.vscode-pull-request-github: 0.112.0
ms-vscode.azure-repos: 0.40.0
github.vscode-github-actions:
0.27.2

VS Code: 1.101.2 (user setup)

OS: Windows 11
VPN: Yes (corporate)

📎 Additional context
The GitHub Enterprise server is reachable in browser (https://git.cedacrigroup.it)
SSO might be enabled on the enterprise server
Tried setting github.gitAuthenticationHosts and github.enterpriseServer in settings.json
Also attempted to manually set github-enterprise.uri

首先，在连接 VPN 的情况下，使用报告中的 GitHub Enterprise 主机，在 Windows 11 上复现 OAuth、device-code 和 PAT 流程。检查 github.gitAuthenticationHosts、github.enterpriseServer 和 github-enterprise.uri 的处理方式，然后验证通过至少一种受支持的流程，是否能够完成对报告中 enterprise server 的登录。

由索引模型根据 Issue 内容生成。