fix(chat): cancel in-flight streaming tool invocations when response is cancelled (#288701)

[maruthang]

What — Pressing Stop while a tool call is still streaming partial input now also transitions that tool invocation to Cancelled, so the "Editing files" label and spinner on the thinking row clear immediately instead of hanging on-screen.

Why — ChatResponseModel.cancel() only flipped its own state. Any ChatToolInvocation that was in StateKind.Streaming (LM still producing tool arguments) stayed there forever. UI consumers (the autorun in chatThinkingContentPart.ts) kept treating the tool as in-progress, so the spinner/shimmer never cleared (#288701).

How — Inside cancel(), walk this._response.value for ChatToolInvocation parts and call the existing cancelFromStreaming(ToolConfirmKind.Skipped) on each. No new API surface. The ToolConfirmKind symbol was already available; just added it to the existing import line.

Test plan — New regression test in chatModel.test.ts (ChatResponseModel suite): create a streaming ChatToolInvocation on a ChatModel, assert state Streaming, call cancelRequest(request), assert state Cancelled and IChatToolInvocation.isComplete true. Manual: trigger a multi-file edit via an agent-mode request, press Stop while the thinking row shows "Editing files"; the label/spinner clears immediately.

Fixes #288701

[connor4312]

thanks!