feat(hooks): add AgentResult to AfterInvocationEvent

[Ratish1]

Description

The AfterInvocationEvent hook event did not provide access to the AgentResult, making it difficult for hooks to perform actions based on the outcome of an agent's invocation. This PR updates the AfterInvocationEvent to include an optional AgentResult. The Agent.invoke method now captures the AgentResult and passes it to the AfterInvocationEvent. New tests have been added to verify the functionality and immutability of the event.

Related Issues

Closes #635

Documentation PR

N/A

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
• I ran formatters, linters and pre-commit checks.
• I ran unit tests:
• test_after_invocation_event_has_optional_result -> passed
• test_after_invocation_event_result_not_writable -> passed
• test_after_invocation_event_agent_not_writable -> passed

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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