Add step notifications in MCP Server response

[Rifdhan]

• Add a new message type for step_notification, which indicates the major steps which the agent is working through while generating its response
• Add test coverage

[gemini-code-assist]

Caution

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[Rifdhan]

@copilot review this change and share feedback but don't make any code changes yourself

[Copilot]

@copilot review this change and share feedback but don't make any code changes yourself

Reviewed the change set end-to-end (types, schema, stream parsing, metrics, and tests) and I don’t see any blocking issues. The step_notification path is wired consistently across runtime/message typing and has good positive/negative test coverage (including metrics + ignored notification variants).