Set user-agent (#2230)

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

codex-rs/core/Cargo.toml

@@ -27,11 +27,12 @@ futures = "0.3"
 libc = "0.2.174"
 mcp-types = { path = "../mcp-types" }
 mime_guess = "2.0"
+os_info = "3.12.0"
 rand = "0.9"
 reqwest = { version = "0.12", features = ["json", "stream"] }
 serde = { version = "1", features = ["derive"] }
-serde_json = "1"
 serde_bytes = "0.11"
+serde_json = "1"
 sha1 = "0.10.6"
 shlex = "1.3.0"
 similar = "2.7.0"
@@ -75,6 +76,7 @@ core_test_support = { path = "tests/common" }
 maplit = "1.0.2"
 predicates = "3"
 pretty_assertions = "1.4.1"
+regex-lite = "0.1.6"
 tempfile = "3"
 tokio-test = "0.4"
 walkdir = "2.5.0"