fix(ai): drop unused ChatMiddlewarePhase import after merge

Surfaced by tsc after merging origin/main (the eslint-config 0.4.0
bump in #607 strengthens unused-import detection). The type is
re-exported elsewhere and not referenced inside this file.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: tombeckenham

packages/typescript/ai/src/activities/chat/index.ts

@@ -55,7 +55,6 @@ import type {
   ChatMiddleware,
   ChatMiddlewareConfig,
   ChatMiddlewareContext,
-  ChatMiddlewarePhase,
   StructuredOutputMiddlewareConfig,
 } from './middleware/types'
 import type { SystemPrompt } from '../../system-prompts'