chore: update spanKind

[poshinchen]

Description

Update the SpanKind following the conventions.

• Mark model_invoke span and start_agent span with SpanKind: CLIENT
• Mark event_loop, tool_call, default to SpanKind.INTERNAL

Documentation PR

Not required

Type of Change

Other (please describe):

• trace span small update

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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