Add plot thumbnail cache feature so plots pane shows thumbnails after a reload

[softwarenerd]

Description

This PR addresses #397 by adding a thumbnail cache feature to plots. Now, when Positron is reloaded, the Plots pane will show thumbnails for all the current plots.

Here's a screen recording of this in action:

Screen.Recording.2025-05-16.at.7.02.52.PM.mov

Tests:
@:plots

Release Notes

New Features

• N/A

Bug Fixes

• Plots pane doesn't show thumbnails after a reload #397 Add plot thumbnail cache feature so plots pane shows thumbnails after a reload.

QA Notes

Please note that #7584 has not been addressed yet. It is a separate bug unrelated to this PR.

[github-actions]

E2E Tests 🚀
This PR will run tests tagged with: @:critical @:plots

^{readme  valid tags}

[Copilot]

Pull Request Overview

This PR adds a thumbnail cache feature so that the plots pane can restore thumbnails after a reload.

• Introduce getCachedPlotThumbnailURI in the service interface and stub in tests
• Implement storage, retrieval, and cleanup of cached thumbnail descriptors in PositronPlotsService
• Update thumbnail components to initialize from cache before the plot renders again

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/vs/workbench/services/positronPlots/test/common/testPositronPlotsService.ts	Added stub getCachedPlotThumbnailURI to the test service
src/vs/workbench/services/positronPlots/common/positronPlots.ts	Added getCachedPlotThumbnailURI to the public service interface
src/vs/workbench/contrib/positronPlots/browser/positronPlotsService.ts	Implemented caching logic: store on save, load on init, and provide retrieval method
src/vs/workbench/contrib/positronPlots/browser/components/webviewPlotThumbnail.tsx	Updated WebviewPlotThumbnail to initialize from cache and subscribe/unsubscribe correctly
src/vs/workbench/contrib/positronPlots/browser/components/dynamicPlotThumbnail.tsx	Updated DynamicPlotThumbnail to initialize from cache (needs cleanup for its listener)

Comments suppressed due to low confidence (2)

src/vs/workbench/services/positronPlots/test/common/testPositronPlotsService.ts:216

• Add unit tests to verify that thumbnail URIs are correctly stored to and retrieved from the cache, including edge cases with missing or invalid entries.

getCachedPlotThumbnailURI(plotId: string) {

src/vs/workbench/contrib/positronPlots/browser/positronPlotsService.ts:96

• [nitpick] Rename thumbnailURI to thumbnailUri to match camelCase conventions used elsewhere (e.g., plotClient.thumbnailUri).

readonly thumbnailURI: string;