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)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/16456).
* #16457
* __->__ #16456

codex-rs/Cargo.lock

@@ -1721,6 +1721,7 @@ dependencies = [
  "clap",
  "codex-client",
  "codex-cloud-tasks-client",
+ "codex-cloud-tasks-mock-client",
  "codex-core",
  "codex-git-utils",
  "codex-login",
@@ -1750,12 +1751,21 @@ dependencies = [
  "chrono",
  "codex-backend-client",
  "codex-git-utils",
- "diffy",
  "serde",
  "serde_json",
  "thiserror 2.0.18",
 ]
 
+[[package]]
+name = "codex-cloud-tasks-mock-client"
+version = "0.0.0"
+dependencies = [
+ "async-trait",
+ "chrono",
+ "codex-cloud-tasks-client",
+ "diffy",
+]
+
 [[package]]
 name = "codex-code-mode"
 version = "0.0.0"