feature(graph): Allow cyclic graphs

[mkmeral]

Description

This PR removes the cyclic graph validation from the multiagent graph implementation, based on customer feedback. Previously, the graph implementation enforced a Directed Acyclic Graph (DAG) structure, which limited certain use cases. By removing this restriction, we now support cyclic graphs, enabling more flexible agent workflows such as:

• "Loop until" patterns where agents can continue processing until a specific condition is met
• Feedback loops where output from later stages can influence earlier stages
• Recursive agent workflows where an agent can feed back into itself
• Iterative refinement processes where results are continuously improved

The changes are minimal and focused on removing the cycle detection algorithm while maintaining all other validation checks.

Related Issues

Addresses customer feedback requesting support for cyclic agent graphs

Documentation PR

TODO

Type of Change

New feature

Testing

• Verified that cyclic graphs can now be created without validation errors
• Updated existing tests to confirm that cyclic graphs are properly supported
• Ran full test suite with hatch test and hatch run test-integ to ensure no regressions
• All tests pass with no new warnings or errors

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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[vria]

Please concider #571