feat(AIC-3106): add multimodal history support to graph().invoke()

[andrewklatzke]

Summary

• Adds an optional history argument to graph().invoke() (and native graph runners), forwarded to the root node only on the first hop — subsequent nodes keep existing string-based data passing.
• Shared compose_history helpers map LaunchDarkly-canonical multimodal content blocks to each provider’s native format (OpenAI Agents, Claude Agents/Messages, LangChain Agents/Messages).
• Adds a graph-history integration example that asserts the model can identify a generated solid-color PNG from history.

Test plan

• Unit tests for client graph plumbing + all handler/native-graph packages
• Integration: uv run python main.py graph-history travel-agent-flow ""
• Full integration suite (all integration-config.json scenarios, success + failure)
• CI green on this PR

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Note

Overview
graph().invoke() now accepts optional history and passes it to the root node only on the first hop; later graph nodes keep string-based threading.

A new client compose_history module (plus exports like content_to_text, image_block_to_url) standardizes how runtime history is ordered and how LD-canonical text/image blocks map to each provider.

Provider handlers stop flattening history into system prompts and instead send structured turns (e.g. Anthropic image blocks with adjacent-user merge, OpenAI input_image, LangChain image_url). Native graph runners (Claude/OpenAI/LangChain) accept history on invoke and seed the root prompt/state only.

Adds a graph-history example (generated red PNG + color check) and solid_color_png_base64 helper; README/main.py document the new example.

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