fix(telemetry): use per-invocation usage in agent span attributes

[en-yao]

Description

In multi-turn sessions, end_agent_span reports accumulated_usage (session-lifetime cumulative tokens) instead of per-invocation usage, causing inflated token metrics in OTEL spans. The per-invocation usage is already tracked at response.metrics.latest_agent_invocation.usage — this PR points end_agent_span to the correct field.

Related Issues

Resolves #2010

Documentation PR

N/A

Type of Change

Bug fix

Testing

Updated existing agent span tests to distinguish between accumulated and per-invocation usage. Added dedicated tests verifying per-invocation values are reported and fallback behavior when no invocations exist.

• 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