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.

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

codex-rs/Cargo.lock

@@ -799,6 +799,7 @@ dependencies = [
  "schemars 0.8.22",
  "serde",
  "serde_json",
+ "shlex",
  "tokio",
  "toml 0.9.1",
  "tracing",