Fix: Detect PR files in #openPullRequest when single file is open

[Copilot]

Problem

The #openPullRequest tool was not detecting PRs where only new files were created. When a newly added file from a PR was opened as a single file (not in a diff view), the tool failed to recognize it as being associated with a PR. This prevented users from asking Copilot about the PR context when viewing these files.

Root Cause

The OpenPullRequestTool._findActivePullRequest() method only checked for TabInputTextDiff (diff editors), but when files with GitChangeType.ADD status are opened in a PR, they open as single files using TabInputText with a Schemes.Pr URI scheme, not as diffs.

Solution

Extended the PR detection logic to also check for TabInputText (single file editors) in addition to the existing TabInputTextDiff support. The fix:

1. Added an else if branch to check for vscode.TabInputText instances
2. Extracts PR parameters from URIs with either:
   • Schemes.Pr - PR files from GitHub (non-checkout PRs)
   • Schemes.Review - Checked out PR files
3. Finds and returns the corresponding pull request model

Changes

• Updated src/lm/tools/openPullRequestTool.ts:
  • Added 16 lines of new logic to handle single file editors
  • Mirrors the existing pattern used for diff editors
  • Maintains consistency with the codebase

Example Scenario

Before: User opens a newly added file newFile.ts in a PR → #openPullRequest shows "No active pull request"

After: User opens a newly added file newFile.ts in a PR → #openPullRequest correctly detects and provides context about the PR

Testing

• ✅ Linting passed
• ✅ Hygiene checks passed
• ✅ Build completed successfully
• ✅ Code follows existing patterns

Fixes the issue where PRs with only newly created files were not detected by the #openPullRequest tool.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Want #openPullRequest to consider PRs where only new files created</issue_title>
<issue_description>

After verifying #7737, I found another variant I'd love to be supported: explaining PRs where the diffs are just newly created files.

I have quite a few PRs in my repo where only new files were added. I'd still love to be able to ask about #openPullRequest if I have a newly added file open.

</issue_description>

<agent_instructions>The problem here is for files, we only detect PR related files when a diff is open. We should also detect that the file is associated with a PR when a PR file scheme file is open, even if not a diff.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

Fixes #7889

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