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/mcp-server/Cargo.toml

@@ -22,6 +22,7 @@ mcp-types = { path = "../mcp-types" }
 schemars = "0.8.22"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
+shlex = "1.3.0"
 toml = "0.9"
 tracing = { version = "0.1.41", features = ["log"] }
 tracing-subscriber = { version = "0.3", features = ["fmt", "env-filter"] }