## 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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* #16457
* __->__ #16456
