Add comprehensive edge case tests for JSON-RPC payload shapes

[Copilot]

Follow-up to #1138: adds tests validating behavior for less-common JSON-RPC payload shapes that may come from various implementations.

New test coverage (26 tests)

• Null property handling: "error": null with result, both result and error null
• Error precedence: when both error and result are present (non-compliant but possible), error takes precedence regardless of property order
• ID edge cases: empty string, zero, negative numbers, long.MaxValue
• Params variations: explicit null, empty object, array (positional arguments)
• Property ordering: unusual orders like params before method, result before id
• String handling: Unicode (CJK, Greek, emoji), escaped characters
• Result types: primitive string, number, boolean, array
• Error data types: null, numeric, array, complex object
• Unknown properties: multiple unknowns interspersed with known properties
• Minimal messages: notification with method only

Example

// Response with explicit null error (lenient handling)
string json = """{"jsonrpc":"2.0","id":1,"result":{"data":"value"},"error":null}""";
var message = JsonSerializer.Deserialize<JsonRpcMessage>(json, McpJsonUtilities.DefaultOptions);
Assert.IsType<JsonRpcResponse>(message); // Treated as success response

Original prompt

As a follow-up to @modelcontextprotocol/csharp-sdk/pull/1138, please add some tests validating behavior for various less-common shapes of JSON-RPC payloads, e.g. a valid message that has an "error": null property. Please try to be thorough in validating various combinations of properties and values while not being overly duplicative. Add the tests to an existing test file if possible.

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