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/core/src/mcp_connection_manager.rs

@@ -18,6 +18,7 @@ use mcp_types::ClientCapabilities;
 use mcp_types::Implementation;
 use mcp_types::Tool;
 
+use serde_json::json;
 use sha1::Digest;
 use sha1::Sha1;
 use tokio::task::JoinSet;
@@ -135,7 +136,9 @@ impl McpConnectionManager {
                                 experimental: None,
                                 roots: None,
                                 sampling: None,
-                                elicitation: None,
+                                // https://modelcontextprotocol.io/specification/2025-06-18/client/elicitation#capabilities
+                                // indicates this should be an empty object.
+                                elicitation: Some(json!({})),
                             },
                             client_info: Implementation {
                                 name: "codex-mcp-client".to_owned(),