## Summary

Fixes #60288 and #60454 .

After a push, the push toast's "Create Pull Request" button fails with
`Unsupported remote URL` when the repository's remote is not a plain,
recognized host. This restores the pre-#53913 behavior as a fallback:
use the create-PR/MR link that `git push` itself prints, and only build
a URL from the provider registry when the push output had no link.

## Why this matters

#53913 made the button always appear and changed `create_pull_request`
to reconstruct the PR URL from the remote via
`git::parse_git_remote_url` against the `GitHostingProviderRegistry`.
When the remote is not recognized, parsing returns `None` and the action
errors. This affects self-hosted GitLab/GitHub, an SSH-config host alias
like `git@personal:owner/repo` (the duplicate #60076), and any
non-standard host. Before #53913, the flow used the link git prints in
the push output, which works regardless of host, so this is a regression
for anyone not pushing to a plainly-recognized GitHub URL.

## Solution

`git push` prints a `remote:` line with the hosting provider's
create-PR/MR URL (GitHub: "Create a pull request for '\<branch>' on
GitHub by visiting:", GitLab: "To create a merge request for \<branch>,
visit:", Bitbucket: "Create pull request for \<branch>:"), and we
already hold the push `RemoteCommandOutput`.

- `remote_output.rs`: add `extract_pull_request_url`, which scans the
push stderr for the first `http(s)` URL on a `remote:` line tied to a
create-PR/MR prompt. It ignores unrelated URLs (for example the OpenSSH
post-quantum warning line).
- `git_panel.rs`: capture that URL in `show_remote_output` into
`pending_pull_request_url`, and prefer it in `create_pull_request`,
falling back to the existing provider construction only when the push
output had no link. The cached URL is consumed once (`take()`) and
cleared when the active repo, the active branch/head, or the pending
remote operation changes, so a later `git: Create Pull Request` action
never opens a stale URL from an earlier push.

Recognized GitHub remotes are unaffected: they still get a link from the
push output, with the provider path as the fallback.

## Testing

Unit tests in `remote_output.rs` cover `extract_pull_request_url` for
the GitHub, GitLab, and Bitbucket prompt formats, the no-link case
(returns `None`, so the provider fallback runs), and an output
containing an unrelated URL that must not be mistaken for the PR link.
The existing remote-operation test also asserts the cached URL is
cleared when a new operation starts.

`cargo test -p git_ui remote_output` passes (5 tests). Tested on macOS.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

---

Release Notes:

- Fixed "Create Pull Request" button in the toast shown after `git:
push` failing for repositories on unrecognized Git hosts by using the
link printed in the push output.
- Fixed the button shown on the toast after `git: push` for GitLab
branches with an existing merge request. It now shows "View Merge
Request" and links to the existing merge request.

---------

Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>
Co-authored-by: dino <dinojoaocosta@gmail.com>