Route asynchronous VS Code responses to their originating panel

[Railly]

Context

The rich extension stores the most recently active panel in shared mutable state. Async handlers build their response function against that shared panel, so a later message from another panel can redirect an earlier response.

Evidence:

• vscode/src/rich-extension.ts:759-789 mutates and reads the shared active view.
• vscode/src/rich-extension.ts:1061-1073 awaits multiple network requests before posting Marketplace content.

Task

Capture the originating WebviewView for every request and keep panel-specific responses separate from broadcasts.

Acceptance criteria

• Async completions return to the panel that initiated them.
• Switching panels during Marketplace loading does not lose or misroute results.
• Broadcast state updates still reach every intended provider.
• A concurrency test interleaves messages from two panels.