Add context and ext fields to all request/response schemas

[bokelley]

Problem

The spec requires:

• context: echo from request to response (correlation IDs, orchestration metadata)
• ext: accept without error on requests (vendor extensions)

But most schemas don't declare these fields. Schema validators reject compliant responses that include context, and strict validators reject requests with ext.

Proposal

• Add context?: ContextObject to every request AND response schema
• Add ext?: object to every request schema

This is declarative — the fields already exist by convention. Making them explicit ensures schema validators accept them.

Discovered during storyboard testing in adcontextprotocol/adcp-client#541 — signals agent failed context echo checks because the response schema didn't declare context.