URI handler rejects PR URLs with underscores in owner name

[muiriswoulfe]

• Extension version: 0.146.0
• VSCode Version: latest stable
• OS: Windows 11
• Repository Clone Configuration: n/a (URI handler invocation, no repo cloned)
• GitHub Product: GitHub.com

The URI handlers /open-pull-request-changes, /checkout-pull-request, and /open-pull-request-webview reject GitHub PR URLs whose owner contains an underscore. Enterprise Managed User (EMU) accounts on github.com receive a _company suffix and therefore contain underscores, so PRs in repositories owned by such accounts cannot be opened via the URI handler.

Steps to Reproduce:

1. Paste into the address bar: vscode://GitHub.vscode-pull-request-github/open-pull-request-changes?uri=https://github.com/some_owner/some-repo/pull/1 (any PR whose owner contains _ will do).
2. Observe the notification "Invalid pull request URI." The equivalent URL from the v0.126.0 changelog (uri=https://github.com/microsoft/vscode-css-languageservice/pull/460) works.

Root cause: validateOpenWebviewParams in src/common/uri.ts validates the owner against ownerRegex = /^(?!-)(?!.*--)[a-zA-Z0-9-]+(?<!-)$/, which excludes _. The companion repoRegex (/^[a-zA-Z0-9_.-]+$/) does allow _, so the two rules are inconsistent.

Fix: relax ownerRegex to allow _, e.g. /^(?!-)(?!.*--)[a-zA-Z0-9_-]+(?<!-)$/.