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

.github/scripts/verify_cargo_workspace_manifests.py

@@ -27,11 +27,6 @@ UTILITY_NAME_EXCEPTIONS = {
     "path-utils": "codex-utils-path",
 }
 MANIFEST_FEATURE_EXCEPTIONS = {
-    "codex-rs/cloud-tasks-client/Cargo.toml": {
-        "default": ("online",),
-        "online": ("dep:codex-backend-client",),
-        "mock": (),
-    },
     "codex-rs/otel/Cargo.toml": {
         "disable-default-metrics-exporter": (),
     },
@@ -43,11 +38,6 @@ MANIFEST_FEATURE_EXCEPTIONS = {
     },
 }
 OPTIONAL_DEPENDENCY_EXCEPTIONS = {
-    (
-        "codex-rs/cloud-tasks-client/Cargo.toml",
-        "dependencies",
-        "codex-backend-client",
-    ),
     (
         "codex-rs/tui/Cargo.toml",
         'target.cfg(not(target_os = "linux")).dependencies',
@@ -55,11 +45,6 @@ OPTIONAL_DEPENDENCY_EXCEPTIONS = {
     ),
 }
 INTERNAL_DEPENDENCY_FEATURE_EXCEPTIONS = {
-    (
-        "codex-rs/cloud-tasks/Cargo.toml",
-        "dependencies",
-        "codex-cloud-tasks-client",
-    ): ("mock", "online"),
     (
         "codex-rs/core/Cargo.toml",
         "dev-dependencies",