fix: reset Run-scoped UI state on selection changes

[pppolf]

What changed

• remount the selected Run workspace when the Run identity changes
• remount the selected Session workspace when the Session identity changes
• add regression coverage proving drafts and uncertain operation state are cleared across both identity transitions
• verify that a draft from Run/Session A cannot be submitted against Run/Session B

Why

React reused the same workspace component instance when only runID or sessionID changed. Run-scoped local state—including unsent message drafts, mutation results, cancellation inputs, and in-memory retry/idempotency state—could therefore survive a sidebar selection change. The newly selected resource supplied the next render's API binding, so an old draft could be accidentally submitted to the wrong Run or Session.

Keying each top-level workspace by its selected identity makes React tear down the old resource scope before rendering the next one. It also aborts/unsubscribes child effects through their normal cleanup paths.

Impact

Switching between Runs or Sessions now starts with a clean resource-scoped UI. Drafts and uncertain local operation state cannot cross the identity boundary.

Validation

Run with the repository CI Node version (Node 24):

• vitest run src/App.test.tsx — 3 passed
• full vitest run — 55 files / 213 tests passed
• tsc -b --pretty false
• npm run check:api
• Vite production build
• git diff --check