Enforce String or Integer non-null ID for MCP JSON-RPC Messages with a new McpSchema subclass

[ZachGerman]

I considered JSONRPCMessageID, but didn't want the JSONRPC prefix as it might imply nullability or possibly a float, as is standard for a JSON-RPC id.
I also considered id but figured it was too ambiguous.
I can see an argument for McpMessageId, but I figured it being in the McpSchema namespace was good enough for someone to identify the MessageId type as specific to MCP.

Motivation and Context

The specification conditions for JSON-RPC Message id values include the requirements of the value being a (non-null) String or Integer.

How Has This Been Tested?

Multiple existing tests use this field, have been updated accordingly, and confirmed to work correctly after the changes.

Breaking Changes

N/A

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

Edit: After sleeping on it, I realized this IS a breaking change, because people are possibly building requests in tool handlers without the requirements this would enforce.

[tzolov]

@ZachGerman if you just want to validate the request ID this looks unnecessarily complicated?

Wouldn't something like this achieve the same:

	@JsonInclude(JsonInclude.Include.NON_ABSENT)
	@JsonIgnoreProperties(ignoreUnknown = true)
	public record JSONRPCRequest( // @formatter:off
		@JsonProperty("jsonrpc") String jsonrpc,
		@JsonProperty("method") String method,
		@JsonProperty("id") Object id,
		@JsonProperty("params") Object params) implements JSONRPCMessage { // @formatter:on

		/**
		 * Constructor that validates MCP-specific ID requirements. Unlike base JSON-RPC,
		 * MCP requires that: (1) Requests MUST include a string or integer ID; (2) The ID
		 * MUST NOT be null
		 */
		public JSONRPCRequest {
			Assert.notNull(id, "MCP requests MUST include an ID - null IDs are not allowed");
			Assert.isTrue(id instanceof String || id instanceof Integer || id instanceof Long,
					"MCP requests MUST have an ID that is either a string or integer");
		}
	}

[tzolov]

@ZachGerman here is how I would address this requirement. Let me know if i'm missing something:
#403

[ZachGerman]

@tzolov nice catch on the Long case!

@ZachGerman here is how I would address this requirement. Let me know if i'm missing something: #403

I think it's great. I just wanted to be sure the folks writing tools are informed of the rules conflicting with base JSON-RPC somewhere along the lines when confronting the types they use to write their tool. In hindsight, this was a bit like taking a flamethrower to an anthill 😅.