Added CoherenceEvaluator

[poshinchen]

Description

Added Coherence evaluator which evaluates the logical cohesion and reasoning quality of agent responses with five-level scoring:

• NOT_AT_ALL = "Not At All"
• NOT_GENERALLY = "Not Generally"
• NEUTRAL_MIXED = "Neutral/Mixed"
• GENERALLY_YES = "Generally Yes"
• COMPLETELY_YES = "Completely Yes"

The evaluator is trace_level, which only contains the last prompt and the response.

Related Issues

#101

Documentation PR

TBD

Type of Change

New feature

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• I ran hatch run prepare

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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