Set user-agent (#2230)

Use the same well-defined value in all cases when sending user-agent
header

codex-rs/Cargo.lock

@@ -689,9 +689,11 @@ dependencies = [
  "mcp-types",
  "mime_guess",
  "openssl-sys",
+ "os_info",
  "predicates",
  "pretty_assertions",
  "rand 0.9.2",
+ "regex-lite",
  "reqwest",
  "seccompiler",
  "serde",
@@ -3044,6 +3046,18 @@ version = "0.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "04744f49eae99ab78e0d5c0b603ab218f515ea8cfe5a456d7629ad883a3b6e7d"
 
+[[package]]
+name = "os_info"
+version = "3.12.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d0e1ac5fde8d43c34139135df8ea9ee9465394b2d8d20f032d38998f64afffc3"
+dependencies = [
+ "log",
+ "plist",
+ "serde",
+ "windows-sys 0.52.0",
+]
+
 [[package]]
 name = "overload"
 version = "0.1.1"