Fix stale resource content in Resources panel

[nicbarrett]

Summary

• render resource details from the selected resource URI instead of a single shared content buffer
• allow explicit resource refreshes to bypass the client cache
• add a regression test for switching back to an already-cached resource

Problem

The Resources panel could show stale content from a previously read resource when switching back to a resource that had already been cached. The selected row and heading updated, but the detail pane could continue showing a different resource's payload.

Root cause

• the Resources tab rendered a single global resourceContent string instead of content derived from selectedResource.uri
• readResource() returned early for cached resources, so selecting an already-read resource did not repopulate the visible pane
• the Refresh button also hit the same cache short-circuit

Fix

• derive displayed resource content from resourceContentMap[selectedResource.uri]
• add a force flag to readResource()
• use the force path from the Resources panel Refresh button

Validation

• npm test --workspace=client -- --runInBand App.resources.test.tsx ResourcesTab.test.tsx

Closes #1266.