ci: block new workspace crate features (#16455)

## Why

We already enforce workspace metadata and lint inheritance for
`codex-rs` manifests, but we still allow new crate features to slip into
the workspace. That makes it too easy to add more Cargo-only feature
permutations while we are trying to eliminate them.

## What changed

- extend `verify_cargo_workspace_manifests.py` to reject new
`[features]` tables in workspace crates
- reject new optional dependencies that create implicit crate features
- reject new workspace-to-workspace `features = [...]` activations and
`default-features = false`
- add a narrow temporary allowlist for the existing feature-bearing
manifests and internal feature activations
- make the allowlist self-shrinking so a follow-up removal has to delete
its corresponding exception


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[//]: # (BEGIN SAPLING FOOTER)
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* #16457
* #16456
* __->__ #16455