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/cloud-tasks/Cargo.toml

@@ -1,8 +1,8 @@
 [package]
-name = "codex-cloud-tasks"
-version.workspace = true
 edition.workspace = true
 license.workspace = true
+name = "codex-cloud-tasks"
+version.workspace = true
 
 [lib]
 name = "codex_cloud_tasks"
@@ -16,32 +16,30 @@ anyhow = { workspace = true }
 base64 = { workspace = true }
 chrono = { workspace = true, features = ["serde"] }
 clap = { workspace = true, features = ["derive"] }
-codex-cloud-tasks-client = { path = "../cloud-tasks-client", features = [
-    "mock",
-    "online",
-] }
 codex-client = { workspace = true }
-codex-core = { path = "../core" }
+codex-cloud-tasks-client = { workspace = true }
+# TODO: codex-cloud-tasks-mock-client should be in dev-dependencies.
+codex-cloud-tasks-mock-client = { workspace = true }
+codex-core = { workspace = true }
 codex-git-utils = { workspace = true }
 codex-login = { path = "../login" }
 codex-tui = { workspace = true }
 codex-utils-cli = { workspace = true }
 crossterm = { workspace = true, features = ["event-stream"] }
+owo-colors = { workspace = true, features = ["supports-colors"] }
 ratatui = { workspace = true }
 reqwest = { workspace = true, features = ["json"] }
 serde = { workspace = true, features = ["derive"] }
 serde_json = { workspace = true }
+supports-color = { workspace = true }
 tokio = { workspace = true, features = ["macros", "rt-multi-thread"] }
 tokio-stream = { workspace = true }
 tracing = { workspace = true, features = ["log"] }
 tracing-subscriber = { workspace = true, features = ["env-filter"] }
 unicode-width = { workspace = true }
-owo-colors = { workspace = true, features = ["supports-colors"] }
-supports-color = { workspace = true }
 
 [dependencies.async-trait]
 workspace = true
 
 [dev-dependencies]
-async-trait = { workspace = true }
 pretty_assertions = { workspace = true }