feat: leverage elicitations in the MCP server (#1623)

This updates the MCP server so that if it receives an
`ExecApprovalRequest` from the `Codex` session, it in turn sends an [MCP
elicitation](https://modelcontextprotocol.io/specification/draft/client/elicitation)
to the client to ask for the approval decision. Upon getting a response,
it forwards the client's decision via `Op::ExecApproval`.

Admittedly, we should be doing the same thing for
`ApplyPatchApprovalRequest`, but this is our first time experimenting
with elicitations, so I'm inclined to defer wiring that code path up
until we feel good about how this one works.

---
[//]: # (BEGIN SAPLING FOOTER)
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* __->__ #1623
* #1622
* #1621
* #1620

codex-rs/mcp-types/src/lib.rs

@@ -931,7 +931,7 @@ pub struct ProgressNotificationParams {
     pub total: Option<f64>,
 }
 
-#[derive(Debug, Clone, PartialEq, Deserialize, Serialize)]
+#[derive(Debug, Clone, PartialEq, Deserialize, Serialize, Hash, Eq)]
 #[serde(untagged)]
 pub enum ProgressToken {
     String(String),
@@ -1031,7 +1031,7 @@ pub struct Request {
     pub params: Option<serde_json::Value>,
 }
 
-#[derive(Debug, Clone, PartialEq, Deserialize, Serialize)]
+#[derive(Debug, Clone, PartialEq, Deserialize, Serialize, Hash, Eq)]
 #[serde(untagged)]
 pub enum RequestId {
     String(String),