[AIC-3451] Register LangChain handlers in the Python example registry

[jeffdupont]

Summary

• Register create_langchain_messages_handler and create_langchain_agents_handler in examples/register.py, matching the TypeScript example registry so providers without a dedicated handler (including Bedrock) resolve through the LangChain wildcard route.
• Add langchain-aws as a dev dependency so that route can construct a Bedrock chat model. graph-streaming against travel-agent-flow was failing with Handler for provider Bedrock not found.

Test plan

• uv lock --check
• make test (1392 passed, 11 skipped)
• Run graph-streaming against travel-agent-flow and confirm it no longer fails with Handler for provider Bedrock not found
  • Live run (staging) exits 0 on this branch, but it also exits 0 on main, so it doesn't exercise the fix: every node in travel-agent-flow on staging currently resolves to OpenAI (orchestrator gpt-5.4, lodging/restaurant/leisure gpt-5, summarizer gpt-5.1), so there's no Bedrock node to reach.
  • Bedrock routing verified offline instead via select_handler with provider Bedrock against the example registry: main raises Handler for provider Bedrock not found for every mode; this branch resolves to the LangChain wildcard handlers (('*', 'messages') / ('*', 'agent')).

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