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`