Consistent JSON forward/backward compatibility (2.0 foundation)

[chemicL]

Motivation and Context

The MCP specification evolves continuously; domain types must absorb
new fields and subtypes without breaking existing clients or servers.
On the 1.x line this is structurally prevented by sealed interfaces,
which make it impossible to add a permitted subtype without breaking
exhaustive pattern-match switch expressions in caller code. This
commit opens the 2.0 release line, where those constraints are lifted
and serialization is made self-contained — independent of any global
ObjectMapper configuration.

Breaking Changes

Breaking changes for users migrating from 1.x

• Sealed interfaces removed from JSONRPCMessage, Request, Result,
  Notification, ResourceContents, CompleteReference and Content.
  Exhaustive switch expressions over these types must add a default
  branch.
• Prompt(name, description, null) no longer silently coerces null
  arguments to an empty list. Use Prompt.withDefaults() to preserve
  the previous behaviour.
• CompleteCompletion.total and .hasMore are now absent from the wire
  when not set, rather than being emitted as null.
• ServerParameters no longer carries Jackson annotations; it is an
  internal configuration class, not a wire type.

What now works that did not before

• CompleteReference polymorphic dispatch (PromptReference vs
  ResourceReference) works through a plain readValue or convertValue
  call — no hand-rolled map inspection required.
• LoggingLevel deserialization is lenient: unknown level strings
  produce null instead of throwing.
• All domain records now tolerate unknown JSON fields, so a client
  built against an older SDK version will not fail when a newer server
  sends fields it does not yet recognise.
• Null optional fields are consistently absent from serialized output
  regardless of ObjectMapper configuration.

Documentation

• CONTRIBUTING adds an "Evolving wire-serialized records" section: a
  9-rule recipe and example for adding a field safely.
• MIGRATION-2.0 documents all breaking changes listed above.

Follow-up coming next

Several spec-required fields (e.g. JSONRPCError.code/message,
ProgressNotification.progress, CreateMessageRequest.maxTokens,
CallToolResult.content) are stored as nullable Java types without a
null guard. If constructed with null, the NON_ABSENT rule silently
omits them, producing invalid wire JSON without throwing. Fix: compact
canonical constructors with Assert.notNull, following the pattern
already in JSONRPCRequest.

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