cloud-tasks: split the mock client out of cloud-tasks-client (#16456)

## Why

`codex-cloud-tasks-client` was mixing two different roles: the real HTTP
client and the mock implementation used by tests and local mock mode.
Keeping both in the same crate forced Cargo feature toggles and Bazel
`crate_features` just to pick an implementation.

This change keeps `codex-cloud-tasks-client` focused on the shared API
surface and real backend client, and moves the mock implementation into
its own crate so we can remove those feature permutations cleanly.

## What changed

- add a new `codex-cloud-tasks-mock-client` crate that owns `MockClient`
- remove the `mock` and `online` features from
`codex-cloud-tasks-client`
- make `codex-cloud-tasks-client` unconditionally depend on
`codex-backend-client` and export `HttpClient` directly
- gate the mock-mode path in `codex-cloud-tasks` behind
`#[cfg(debug_assertions)]`, so release builds always initialize the real
HTTP client
- update `codex-cloud-tasks` and its tests to use
`codex-cloud-tasks-mock-client::MockClient` wherever mock behavior is
needed
- remove the matching Bazel `crate_features` override and shrink the
manifest verifier allowlist accordingly

## How tested

- `cargo test -p codex-cloud-tasks-client`
- `cargo test -p codex-cloud-tasks-mock-client`
- `cargo test -p codex-cloud-tasks`

---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/16456).
* #16457
* __->__ #16456

codex-rs/Cargo.toml

@@ -18,6 +18,7 @@ members = [
     "cloud-requirements",
     "cloud-tasks",
     "cloud-tasks-client",
+    "cloud-tasks-mock-client",
     "cli",
     "connectors",
     "config",
@@ -96,10 +97,9 @@ license = "Apache-2.0"
 [workspace.dependencies]
 # Internal
 app_test_support = { path = "app-server/tests/common" }
-codex-ansi-escape = { path = "ansi-escape" }
 codex-analytics = { path = "analytics" }
+codex-ansi-escape = { path = "ansi-escape" }
 codex-api = { path = "codex-api" }
-codex-code-mode = { path = "code-mode" }
 codex-app-server = { path = "app-server" }
 codex-app-server-client = { path = "app-server-client" }
 codex-app-server-protocol = { path = "app-server-protocol" }
@@ -112,16 +112,19 @@ codex-chatgpt = { path = "chatgpt" }
 codex-cli = { path = "cli" }
 codex-client = { path = "codex-client" }
 codex-cloud-requirements = { path = "cloud-requirements" }
-codex-connectors = { path = "connectors" }
+codex-cloud-tasks-client = { path = "cloud-tasks-client" }
+codex-cloud-tasks-mock-client = { path = "cloud-tasks-mock-client" }
+codex-code-mode = { path = "code-mode" }
 codex-config = { path = "config" }
+codex-connectors = { path = "connectors" }
 codex-core = { path = "core" }
 codex-core-skills = { path = "core-skills" }
 codex-exec = { path = "exec" }
 codex-exec-server = { path = "exec-server" }
 codex-execpolicy = { path = "execpolicy" }
 codex-experimental-api-macros = { path = "codex-experimental-api-macros" }
-codex-feedback = { path = "feedback" }
 codex-features = { path = "features" }
+codex-feedback = { path = "feedback" }
 codex-file-search = { path = "file-search" }
 codex-git-utils = { path = "git-utils" }
 codex-hooks = { path = "hooks" }
@@ -137,9 +140,9 @@ codex-otel = { path = "otel" }
 codex-plugin = { path = "plugin" }
 codex-process-hardening = { path = "process-hardening" }
 codex-protocol = { path = "protocol" }
-codex-rollout = { path = "rollout" }
 codex-responses-api-proxy = { path = "responses-api-proxy" }
 codex-rmcp-client = { path = "rmcp-client" }
+codex-rollout = { path = "rollout" }
 codex-sandboxing = { path = "sandboxing" }
 codex-secrets = { path = "secrets" }
 codex-shell-command = { path = "shell-command" }
@@ -150,7 +153,6 @@ codex-stdio-to-uds = { path = "stdio-to-uds" }
 codex-terminal-detection = { path = "terminal-detection" }
 codex-tools = { path = "tools" }
 codex-tui = { path = "tui" }
-codex-v8-poc = { path = "v8-poc" }
 codex-utils-absolute-path = { path = "utils/absolute-path" }
 codex-utils-approval-presets = { path = "utils/approval-presets" }
 codex-utils-cache = { path = "utils/cache" }
@@ -171,8 +173,9 @@ codex-utils-rustls-provider = { path = "utils/rustls-provider" }
 codex-utils-sandbox-summary = { path = "utils/sandbox-summary" }
 codex-utils-sleep-inhibitor = { path = "utils/sleep-inhibitor" }
 codex-utils-stream-parser = { path = "utils/stream-parser" }
-codex-utils-template = { path = "utils/template" }
 codex-utils-string = { path = "utils/string" }
+codex-utils-template = { path = "utils/template" }
+codex-v8-poc = { path = "v8-poc" }
 codex-windows-sandbox = { path = "windows-sandbox-rs" }
 core_test_support = { path = "core/tests/common" }
 mcp_test_support = { path = "mcp-server/tests/common" }
@@ -217,12 +220,12 @@ gethostname = "1.1.0"
 globset = "0.4"
 hmac = "0.12.1"
 http = "1.3.1"
+iana-time-zone = "0.1.64"
 icu_decimal = "2.1"
 icu_locale_core = "2.1"
 icu_provider = { version = "2.1", features = ["sync"] }
 ignore = "0.4.23"
 image = { version = "^0.25.9", default-features = false }
-iana-time-zone = "0.1.64"
 include_dir = "0.7.4"
 indexmap = "2.12.0"
 insta = "1.46.3"
@@ -264,7 +267,6 @@ regex-lite = "0.1.8"
 reqwest = "0.12"
 rmcp = { version = "0.15.0", default-features = false }
 runfiles = { git = "https://github.com/dzbarsky/rules_rust", rev = "b56cbaa8465e74127f1ea216f813cd377295ad81" }
-v8 = "=146.4.0"
 rustls = { version = "0.23", default-features = false, features = [
     "ring",
     "std",
@@ -333,6 +335,7 @@ unicode-width = "0.2"
 url = "2"
 urlencoding = "2.1"
 uuid = "1"
+v8 = "=146.4.0"
 vt100 = "0.16.2"
 walkdir = "2.5.0"
 webbrowser = "1.0"