Fix create pull request failing for remotes with no provider

[fly2xj]

Summary

The post-push toast's "Create Pull Request" button regressed in #53913: it now always dispatches the git::CreatePullRequest action, which parses the remote URL through the GitHostingProviderRegistry. For remotes whose host has no registered provider (e.g. internal GitLab instances such as git.woa.com), parse_git_remote_url returns None and the button fails with:

Unsupported remote URL: git@git.woa.com:ybtm-client/dtm-harness.git
Root cause

Before #53913, the "Create Pull Request" button only appeared when git's push output contained a create-PR/MR link (e.g. GitLab's remote: To create a merge request for ..., visit: ), and clicking it opened that URL directly via cx.open_url(&link) — no remote-URL parsing, so it worked for any host.

#53913 changed the button to always appear and to dispatch git::CreatePullRequest, which routes through create_pull_request() → git::parse_git_remote_url(). That walks Zed's hosting-provider registry, and no provider matches hosts like git.woa.com (an internal GitLab instance; GitLab's self-hosted detection requires host.contains("gitlab")). So parse_git_remote_url returns None and the error is surfaced to the user.

Fix

Restore the stderr-link path alongside the new always-show button, preferring git's own output when present:

remote_output.rs: re-added a PushPrLink { link } SuccessStyle variant and an extract_pull_request_link helper that scans only remote:-prefixed lines (so unrelated links such as OpenSSH's post-quantum warning are ignored). This works for any host because the server produces the URL itself.
git_panel.rs: when PushPrLink is produced, the "Create Pull Request" button opens that URL directly; otherwise it falls back to dispatching CreatePullRequest (the provider-based path, for known hosts without a stderr link). The ToastWithLog "View Log" fallback for non-push actions is preserved.
Re-added the linkify dependency to git_ui.
Testing

Updated the three existing push tests to assert PushPrLink with the correct extracted URLs.
Added test_push_internal_host_merge_request_link — a regression guard mirroring the reported git.woa.com scenario.
All 116 git_ui lib tests pass; ./script/clippy -p git_ui and cargo fmt are clean.
Release Notes:

Fixed the "Create Pull Request" button on the post-push toast failing with "Unsupported remote URL" for remotes whose host has no registered git hosting provider (e.g. internal GitLab instances).

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