feat(core): add SEP-2575 request metadata and discovery types

[Yusufihsangorgel]

Summary

Part of #815.

Adds the core wire surface needed before request-scoped runtime behavior can be implemented:

• typed, experimental accessors for the 2026 request metadata while preserving unknown extensions and the legacy request shape;
• server/discover, its required cache fields, and unsupported-version error data; and
• polymorphic request/result decoding with exact wire-format and malformed-input coverage.

Motivation and Context

SEP-2575 moves protocol and client context onto individual requests. The SDK needs a dual-era representation before admission and dispatch can enforce the modern contract without breaking legacy requests.

This PR deliberately stops at the schema/codec boundary. It does not add server dispatch, modern request admission, Streamable HTTP, authentication, subscriptions, MRTR, or the general cache contract.

How Has This Been Tested?

• JVM: 590 tests
• JS and WasmJS: 521 tests per target on Node and Chrome
• iOS Simulator arm64 and macOS arm64: 521 tests per target
• API check and configured KMP API compilation
• ktlint and detekt

A full :kotlin-sdk-core:build was also attempted. It stops locally at tvosSimulatorArm64Test because the Kotlin/Xcode environment cannot run that simulator target. The same task fails with the same device-property error on an exact clean main worktree.

Breaking Changes

This adds DiscoverRequest as a new ClientRequest subtype. Consumers recompiling exhaustive when expressions over ClientRequest may need to handle it. Existing request wire shapes and runtime behavior are unchanged.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

The discovery defaults are always encoded because the draft schema requires them even when a caller uses a Json instance with defaults disabled. New API is marked experimental while the 2026-07-28 specification remains a draft.