app-server: move transport into dedicated crate (#20545)

## Why

`codex-app-server` currently owns both request-processing code and
transport implementation details. Splitting the transport layer into its
own crate makes that boundary explicit, reduces the amount of
transport-specific dependency surface carried by `codex-app-server`, and
gives future transport work a narrower place to evolve.

## What changed

- Added `codex-app-server-transport` and moved the existing transport
tree into it, including stdio, unix socket, websocket, remote-control
transport, and websocket auth.
- Moved shared transport-facing message types into the new crate so both
the transport implementation and `codex-app-server` use the same
definitions.
- Kept processor-facing connection state and outbound routing in
`codex-app-server`, with the routing tests moved next to that local
wrapper.
- Updated workspace metadata, Bazel crate metadata, and
`codex-app-server` dependencies for the new crate boundary.

## Validation

- `cargo metadata --locked --no-deps`
- `git diff --check`
- Attempted `cargo test -p codex-app-server-transport`, `cargo test -p
codex-app-server`, `just fix -p codex-app-server-transport`, and `just
fix -p codex-app-server`; all were blocked before compilation by the
existing `packageproxy` resolution failure for locked `rustls-webpki =
0.103.13`.
- Attempted Bazel build / lockfile validation; those were blocked by
external fetch failures against BuildBuddy / GitHub while resolving
`v8`.

codex-rs/app-server-transport/Cargo.toml

@@ -0,0 +1,58 @@
+[package]
+name = "codex-app-server-transport"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[lib]
+name = "codex_app_server_transport"
+path = "src/lib.rs"
+
+[lints]
+workspace = true
+
+[dependencies]
+anyhow = { workspace = true }
+axum = { workspace = true, default-features = false, features = [
+    "http1",
+    "json",
+    "tokio",
+    "ws",
+] }
+base64 = { workspace = true }
+clap = { workspace = true, features = ["derive"] }
+codex-api = { workspace = true }
+codex-app-server-protocol = { workspace = true }
+codex-core = { workspace = true }
+codex-login = { workspace = true }
+codex-model-provider = { workspace = true }
+codex-state = { workspace = true }
+codex-uds = { workspace = true }
+codex-utils-absolute-path = { workspace = true }
+codex-utils-rustls-provider = { workspace = true }
+constant_time_eq = { workspace = true }
+futures = { workspace = true }
+gethostname = { workspace = true }
+hmac = { workspace = true }
+jsonwebtoken = { workspace = true }
+owo-colors = { workspace = true, features = ["supports-colors"] }
+serde = { workspace = true, features = ["derive"] }
+serde_json = { workspace = true }
+sha2 = { workspace = true }
+time = { workspace = true }
+tokio = { workspace = true, features = [
+    "io-std",
+    "macros",
+    "rt-multi-thread",
+] }
+tokio-tungstenite = { workspace = true }
+tokio-util = { workspace = true }
+tracing = { workspace = true, features = ["log"] }
+url = { workspace = true }
+uuid = { workspace = true, features = ["serde", "v7"] }
+
+[dev-dependencies]
+chrono = { workspace = true }
+codex-config = { workspace = true }
+pretty_assertions = { workspace = true }
+tempfile = { workspace = true }