fix(core): emit correct agent in agent_updated_stream_event during streaming handoff

[vrtnis]

This change corrects a bug where the agent_updated_stream_event was emitting the stale currentAgent rather than the newly active agent after a handoff.

• Updates run.ts to emit result.state._currentAgent
• Adds a streaming test to verify the new agent is emitted
• Includes a patch-level changeset for @openai/agents-core

Resolves #202

[changeset-bot]

🦋 Changeset detected

Latest commit: 342d3a6

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 5 packages

Name	Type
@openai/agents-core	Patch
@openai/agents-openai	Patch
@openai/agents-realtime	Patch
@openai/agents	Patch
@openai/agents-extensions	Patch

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[seratch]

Thank you so much for fixing this! Having the corresponding unit test is appreciated too!