fix: chat functionality and extended thinking handling

- bundle/bundle.md: Add provider-anthropic with source URL and orchestrator config override
  to disable extended_thinking (foundation enables it by default, causes empty text
  responses for simple queries)
- Dockerfile: Add amplifier-module-provider-anthropic pip install for container
- backend/agui_adapter.py: Fix thinking block handling and empty text block rendering
  - Thinking blocks no longer fall through to text handler
  - Text blocks only emit START when actual content arrives
  - Handle amplifierd wire format (block_index/block_type, complete text in content_block:end)
- frontend/src/hooks/useChat.ts: Reset assistantId on TEXT_MESSAGE_END to prevent stale
  message IDs across turns
- tests/test_agui_adapter.py: Add extended thinking test coverage

Chat functionality now works end-to-end. Verified: login -> create session -> send message -> receive AI response visible in chat UI.

Generated with Amplifier

bundle/bundle.md

@@ -4,6 +4,17 @@ bundle:
   version: 0.1.0
   description: General-purpose AI research tool with browser control and artifact generation
 
+providers:
+  - module: provider-anthropic
+    source: git+https://github.com/microsoft/amplifier-module-provider-anthropic@main
+    config:
+      default_model: claude-sonnet-4-20250514
+
+orchestrator:
+  module: loop-streaming
+  config:
+    extended_thinking: false
+
 includes:
   - bundle: git+https://github.com/microsoft/amplifier-foundation@main
   - bundle: research-workbench:behaviors/research-workbench