Switch over to multi-panel

[alexr00]

Fixes #3058

[Copilot]

Pull request overview

This PR addresses #3058 by changing the PR/issue overview webview implementation to support multiple concurrently open description panels instead of reusing a single shared panel.

Changes:

• Replace single currentPanel tracking with a per-identity _panels: Map<string, Panel> keyed by owner/repo#number.
• Update PR overview APIs (getCurrentPullRequest, getActivePanel, findPanel) to work with the multi-panel model.
• Update PR overview tests to validate “reuse for same PR” and “separate panels for different PRs” behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/github/issueOverview.ts	Introduces panelKey and multi-panel tracking/lookup for issue/PR overview panels; updates dispose cleanup to remove from the map.
src/github/pullRequestOverview.ts	Adapts PR overview to the shared multi-panel infrastructure and delegates active/lookup behavior to the base implementation.
src/test/github/pullRequestOverview.test.ts	Updates test expectations and adds coverage for multiple PR panels coexisting.

Comments suppressed due to low confidence (1)

src/test/github/pullRequestOverview.test.ts:15

• Unused import panelKey (not referenced anywhere in this test file). This will fail eslint @typescript-eslint/no-unused-vars; please remove it.

import { panelKey } from '../../github/issueOverview';
import { PullRequestModel } from '../../github/pullRequestModel';