T4 - add request metadata, result type, cache hints, error codes, discover schemas types

Target branch: 2026-07-28-protocol-support

• MetaKeys object with the reserved _meta keys (progressToken, io.modelcontextprotocol/protocolVersion,
  clientInfo, clientCapabilities, logLevel, subscriptionId, serverInfo, traceparent, tracestate,
  baggage) and helpers to read and write typed values in Option[Map[String, Json]].
• ResultType (Complete, InputRequired, Other(String)) and resultType: Option[ResultType] on every result type.
• CacheScope (Public, Private) and ttlMs, cacheScope on ListToolsResponse, ListPromptsResult,
  ListResourcesResult, ListResourceTemplatesResult, ReadResourceResult.
• DiscoverParams and DiscoverResult for server/discover.
• Error codes -32020, -32021, -32022 with typed data (supported, requested, requiredCapabilities).
• extensions: Option[Map[String, Json]] on ClientCapabilities and ServerCapabilities.
• Schema test cases for each type against the 2026-07-28 schema.

Spec:

• Basic - _meta
• Basic - Result Responses
• Basic - Error Codes
• Caching - Cacheable Results
• Discover - DiscoverResult
• Versioning - Extension Negotiation
• Schema - RequestMetaObject
• Schema - CacheableResult
• Schema - ServerCapabilities

Conformance scenarios that must pass after this task: none.

Docs: no.

Read the linked 2026-07-28 specification sections first, then locate the existing result, capability, error, and schema-test types in the Scala project. Implement the listed metadata, result, caching, discovery, error, and extension additions, and add schema test cases for each type against the 2026-07-28 schema.