cloud-tasks: remove cloud-tasks-client crate features

## Why

`codex-cloud-tasks-client` was still using Cargo features to select its HTTP and mock implementations. That leaks feature permutations into both Cargo and Bazel for a crate that does not need them.

## What changed

- delete the `online` and `mock` crate features from `codex-cloud-tasks-client`
- make `codex-backend-client` unconditional and always export both `HttpClient` and `MockClient`
- remove the internal `features = ["mock", "online"]` activation from `codex-cloud-tasks`
- remove the matching Bazel `crate_features` override
- shrink the manifest verifier allowlist to match the remaining exceptions

.github/scripts/verify_cargo_workspace_manifests.py

@@ -25,11 +25,6 @@ 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": (),
     },
@@ -41,11 +36,6 @@ 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',
@@ -53,11 +43,6 @@ 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",