fix: mcp-types serialization wasn't quite working (#791)

While creating a basic MCP server in
https://github.com/openai/codex/pull/792, I discovered a number of bugs
with the initial `mcp-types` crate that I needed to fix in order to
implement the server.

For example, I discovered that when serializing a message, `"jsonrpc":
"2.0"` was not being included.

I changed the codegen so that the field is added as:

```rust
    #[serde(rename = "jsonrpc", default = "default_jsonrpc")]
    pub jsonrpc: String,
```

This ensures that the field is serialized as `"2.0"`, though the field
still has to be assigned, which is tedious. I may experiment with
`Default` or something else in the future. (I also considered creating a
custom serializer, but I'm not sure it's worth the trouble.)

While here, I also added `MCP_SCHEMA_VERSION` and `JSONRPC_VERSION` as
`pub const`s for the crate.

I also discovered that MCP rejects sending `null` for optional fields,
so I had to add `#[serde(skip_serializing_if = "Option::is_none")]` on
`Option` fields.

---
[//]: # (BEGIN SAPLING FOOTER)
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* #792
* __->__ #791

codex-rs/mcp-types/tests/initialize.rs

@@ -5,6 +5,7 @@ use mcp_types::InitializeRequestParams;
 use mcp_types::JSONRPCMessage;
 use mcp_types::JSONRPCRequest;
 use mcp_types::RequestId;
+use mcp_types::JSONRPC_VERSION;
 use serde_json::json;
 
 #[test]
@@ -30,6 +31,7 @@ fn deserialize_initialize_request() {
     };
 
     let expected_req = JSONRPCRequest {
+        jsonrpc: JSONRPC_VERSION.into(),
         id: RequestId::Integer(1),
         method: "initialize".into(),
         params: Some(json!({