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/Cargo.toml

@@ -30,7 +30,6 @@ axum = { workspace = true, default-features = false, features = [
     "ws",
 ] }
 codex-analytics = { workspace = true }
-codex-api = { workspace = true }
 codex-arg0 = { workspace = true }
 codex-cloud-requirements = { workspace = true }
 codex-config = { workspace = true }
@@ -58,6 +57,7 @@ codex-model-provider = { workspace = true }
 codex-models-manager = { workspace = true }
 codex-protocol = { workspace = true }
 codex-app-server-protocol = { workspace = true }
+codex-app-server-transport = { workspace = true }
 codex-feedback = { workspace = true }
 codex-rmcp-client = { workspace = true }
 codex-rollout = { workspace = true }
@@ -65,18 +65,11 @@ codex-sandboxing = { workspace = true }
 codex-state = { workspace = true }
 codex-thread-store = { workspace = true }
 codex-tools = { workspace = true }
-codex-uds = { workspace = true }
 codex-utils-absolute-path = { workspace = true }
 codex-utils-json-to-toml = { workspace = true }
-codex-utils-rustls-provider = { workspace = true }
 chrono = { workspace = true }
 clap = { workspace = true, features = ["derive"] }
-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 }
@@ -93,7 +86,6 @@ tokio = { workspace = true, features = [
     "signal",
 ] }
 tokio-util = { workspace = true }
-tokio-tungstenite = { workspace = true }
 tracing = { workspace = true, features = ["log"] }
 tracing-subscriber = { workspace = true, features = ["env-filter", "fmt", "json"] }
 url = { workspace = true }
@@ -111,6 +103,7 @@ core_test_support = { workspace = true }
 codex-model-provider-info = { workspace = true }
 codex-utils-cargo-bin = { workspace = true }
 flate2 = { workspace = true }
+hmac = { workspace = true }
 opentelemetry = { workspace = true }
 opentelemetry_sdk = { workspace = true }
 pretty_assertions = { workspace = true }