feat(agents): Add get_config() for RunnableConfig access in middleware

[aran-yogesh]

Resolves #33726

• Add get_config() function to access RunnableConfig in middleware hooks
• Enable context-aware middleware based on metadata, tags, and config
• Add comprehensive test suite with 9 unit tests
• Update documentation with usage examples
• Maintain full backward compatibility

This allows middleware to implement user-specific behavior, conditional logging, audit trails, and environment-specific processing based on the RunnableConfig passed to agent.invoke().

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