feat: expose usage and metrics in AfterModelCallEvent

[svtadayon]

Description

Currently, per-call model invocation metrics are not exposed unless OpenTelemetry tracing is enabled. Instead, aggregated metrics are available at the end of agent invocation. This PR exposes usage and metrics through the ModelStopResponse class. Adding these metrics for each model call rather than the aggregation is valuable for SDK users as it enables improved cost optimization.

The fields have been implemented as optional for backwards compatibility.

Related Issues

#1503
#1532
#1575 <- adds these fields to MessageAddedEvent; I would argue that they're more appropriate on the AfterModelCallEvent but I'd be happy to see that PR merged too

Documentation PR

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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