feat: add websocket auth for app-server (#14847)

## Summary
This change adds websocket authentication at the app-server transport
boundary and enforces it before JSON-RPC `initialize`, so authenticated
deployments reject unauthenticated clients during the websocket
handshake rather than after a connection has already been admitted.

During rollout, websocket auth is opt-in for non-loopback listeners so
we do not break existing remote clients. If `--ws-auth ...` is
configured, the server enforces auth during websocket upgrade. If auth
is not configured, non-loopback listeners still start, but app-server
logs a warning and the startup banner calls out that auth should be
configured before real remote use.

The server supports two auth modes: a file-backed capability token, and
a standard HMAC-signed JWT/JWS bearer token verified with the
`jsonwebtoken` crate, with optional issuer, audience, and clock-skew
validation. Capability tokens are normalized, hashed, and compared in
constant time. Short shared secrets for signed bearer tokens are
rejected at startup. Requests carrying an `Origin` header are rejected
with `403` by transport middleware, and authenticated clients present
credentials as `Authorization: Bearer <token>` during websocket upgrade.

## Validation
- `cargo test -p codex-app-server transport::auth`
- `cargo test -p codex-cli app_server_`
- `cargo clippy -p codex-app-server --all-targets -- -D warnings`
- `just bazel-lock-check`

Note: in the broad `cargo test -p codex-app-server
connection_handling_websocket` run, the touched websocket auth cases
passed, but unrelated Unix shutdown tests failed with a timeout in this
environment.

---------

Co-authored-by: Eric Traut <etraut@openai.com>

codex-rs/Cargo.toml

@@ -202,6 +202,7 @@ chrono = "0.4.43"
 clap = "4"
 clap_complete = "4"
 color-eyre = "0.6.3"
+constant_time_eq = "0.3.1"
 crossbeam-channel = "0.5.15"
 crossterm = "0.28.1"
 csv = "1.3.1"
@@ -220,6 +221,7 @@ flate2 = "1.1.4"
 futures = { version = "0.3", default-features = false }
 gethostname = "1.1.0"
 globset = "0.4"
+hmac = "0.12.1"
 http = "1.3.1"
 icu_decimal = "2.1"
 icu_locale_core = "2.1"
@@ -232,6 +234,7 @@ indexmap = "2.12.0"
 insta = "1.46.3"
 inventory = "0.3.19"
 itertools = "0.14.0"
+jsonwebtoken = "9.3.1"
 keyring = { version = "3.6", default-features = false }
 landlock = "0.4.4"
 lazy_static = "1"