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/Cargo.toml

@@ -8,6 +8,7 @@ members = [
     "ansi-escape",
     "async-utils",
     "app-server",
+    "app-server-transport",
     "app-server-client",
     "app-server-protocol",
     "app-server-test-client",
@@ -127,6 +128,7 @@ codex-ansi-escape = { path = "ansi-escape" }
 codex-api = { path = "codex-api" }
 codex-aws-auth = { path = "aws-auth" }
 codex-app-server = { path = "app-server" }
+codex-app-server-transport = { path = "app-server-transport" }
 codex-app-server-client = { path = "app-server-client" }
 codex-app-server-protocol = { path = "app-server-protocol" }
 codex-app-server-test-client = { path = "app-server-test-client" }