chore: update LaunchDarkly AI SDK dependencies to latest versions

[kinyoklion]

Summary

Updates the LaunchDarkly AI SDK dependency version specs across all per-example pyproject.toml files to allow the latest releases:

• launchdarkly-server-sdk-ai: ^0.17.0 → ^0.18.0 (all 8 examples)
• launchdarkly-server-sdk-ai-langchain: ^0.4.0 → ^0.5.0 (langchain, langgraph_agent, langgraph_multi_agent, judge)
• launchdarkly-server-sdk-ai-openai: ^0.3.0 → ^0.4.0 (openai, judge)

The previous ^ ranges did not cover the latest minor versions (e.g. ^0.17.0 only allows >=0.17.0, <0.18.0 in Poetry).

Review & Testing Checklist for Human

• Run cd examples/openai && poetry install && poetry run openai-example with valid SDK and OpenAI keys
• Run cd examples/langchain && poetry install && poetry run langchain-example to verify LangChain integration

Notes

Version spec update only; no code changes.

Link to Devin session: https://app.devin.ai/sessions/2235e3cbcafe4639bed49daf5aa0d506
Requested by: @kinyoklion

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