otel: remove the last workspace crate feature (#16469)

## Why

`codex-otel` still carried `disable-default-metrics-exporter`, which was
the last remaining workspace crate feature.

We are removing workspace crate features because they do not fit our
current build model well:

- our Bazel setup does not honor crate features today, which can let
feature-gated issues go unnoticed
- they create extra crate build permutations that we want to avoid

For this case, the feature was only being used to keep the built-in
Statsig metrics exporter off in test and debug-oriented contexts. This
repo already treats `debug_assertions` as the practical proxy for that
class of behavior, so OTEL should follow the same convention instead of
keeping a dedicated crate feature alive.

## What changed

- removed `disable-default-metrics-exporter` from
`codex-rs/otel/Cargo.toml`
- removed the `codex-otel` dev-dependency feature activation from
`codex-rs/core/Cargo.toml`
- changed `codex-rs/otel/src/config.rs` so the built-in
`OtelExporter::Statsig` default resolves to `None` when
`debug_assertions` is enabled, with a focused unit test covering that
behavior
- removed the final feature exceptions from
`.github/scripts/verify_cargo_workspace_manifests.py`, so workspace
crate features are now hard-banned instead of temporarily allowlisted
- expanded the verifier error message to explain the Bazel mismatch and
build-permutation cost behind that policy

## How tested

- `python3 .github/scripts/verify_cargo_workspace_manifests.py`
- `cargo test -p codex-otel`
- `cargo test -p codex-core
metrics_exporter_defaults_to_statsig_when_missing`
- `cargo test -p codex-app-server app_server_default_analytics_`
- `just bazel-lock-check`

.github/scripts/verify_cargo_workspace_manifests.py

@@ -26,19 +26,9 @@ TOP_LEVEL_NAME_EXCEPTIONS = {
 UTILITY_NAME_EXCEPTIONS = {
     "path-utils": "codex-utils-path",
 }
-MANIFEST_FEATURE_EXCEPTIONS = {
-    "codex-rs/otel/Cargo.toml": {
-        "disable-default-metrics-exporter": (),
-    },
-}
+MANIFEST_FEATURE_EXCEPTIONS = {}
 OPTIONAL_DEPENDENCY_EXCEPTIONS = set()
-INTERNAL_DEPENDENCY_FEATURE_EXCEPTIONS = {
-    (
-        "codex-rs/core/Cargo.toml",
-        "dev-dependencies",
-        "codex-otel",
-    ): ("disable-default-metrics-exporter",),
-}
+INTERNAL_DEPENDENCY_FEATURE_EXCEPTIONS = {}
 
 
 def main() -> int:
@@ -73,6 +63,12 @@ def main() -> int:
         "opt into workspace lints, and avoid introducing new workspace crate "
         "features."
     )
+    print(
+        "Workspace crate features are disallowed because our Bazel build setup "
+        "does not honor them today, which can let issues hidden behind feature "
+        "gates go unnoticed, and because they add extra crate build "
+        "permutations we want to avoid."
+    )
     print(
         "Cargo only applies `codex-rs/Cargo.toml` `[workspace.lints.clippy]` "
         "entries to a crate when that crate declares:"
@@ -91,8 +87,8 @@ def main() -> int:
         "`codex-rs/utils/<crate>/Cargo.toml`."
     )
     print(
-        "Existing workspace crate features are temporarily allowlisted in this "
-        "script and must shrink as they are removed."
+        "Workspace crate features are forbidden; add a targeted exception here "
+        "only if there is a deliberate temporary migration in flight."
     )
     print()
     for path in sorted(failures_by_path):