fix(rmcp): preserve elicitation requestedSchema $schema dialect (#1176)

* fix(rmcp): preserve elicitation requestedSchema $schema dialect

ElicitationSchema round-trips through ElicitationSchemaWire, which had no
field for the top-level `$schema` keyword. A dialect declared on an incoming
requestedSchema (allowed since protocol revision 2025-11-25) was therefore
accepted but silently dropped on re-serialization.

Add a `schema` field to both the public struct and the wire bridge, thread it
through the two `From` conversions, and expose a `with_schema` setter. The
field is `skip_serializing_if = "Option::is_none"`, so schemas without a
dialect serialize unchanged.

Refs #1168

* test(rmcp): regenerate server message schema snapshot for elicitation $schema

The $schema dialect field added to ElicitationSchema changes the
schemars-generated ServerJsonRpcMessage schema. Regenerate the golden
snapshot so test_server_json_rpc_message_schema passes.

* fix(rmcp): generate elicitation $schema as optional string in schemars

Use schemars(with = "String") on ElicitationSchema::$schema so
schemars generates "type": "string" instead of ["string", "null"].

crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json

@@ -963,6 +963,10 @@
       "description": "Type-safe elicitation schema for requesting structured user input.\n\nThis enforces the MCP 2025-06-18 specification that elicitation schemas\nmust be objects with primitive-typed properties.\n\n# Example\n\n```rust\nuse rmcp::model::*;\n\nlet schema = ElicitationSchema::builder()\n    .required_email(\"email\")\n    .required_integer(\"age\", 0, 150)\n    .optional_bool(\"newsletter\", false)\n    .build();\n```",
       "type": "object",
       "properties": {
+        "$schema": {
+          "description": "Optional JSON Schema dialect identifier (the `$schema` keyword).\n\nThe 2025-11-25 protocol revision allows a `requestedSchema` to declare its\ndialect. It is preserved verbatim so a declared dialect survives a\ndecode/re-encode round-trip instead of being silently dropped.",
+          "type": "string"
+        },
         "description": {
           "description": "Optional description of what this schema represents",
           "type": [