Remove test-support feature from codex-core and replace it with explicit test toggles (#11405)

## Why

`codex-core` was being built in multiple feature-resolved permutations
because test-only behavior was modeled as crate features. For a large
crate, those permutations increase compile cost and reduce cache reuse.

## Net Change

- Removed the `test-support` crate feature and related feature wiring so
`codex-core` no longer needs separate feature shapes for test consumers.
- Standardized cross-crate test-only access behind
`codex_core::test_support`.
- External test code now imports helpers from
`codex_core::test_support`.
- Underlying implementation hooks are kept internal (`pub(crate)`)
instead of broadly public.

## Outcome

- Fewer `codex-core` build permutations.
- Better incremental cache reuse across test targets.
- No intended production behavior change.

codex-rs/app-server/tests/common/Cargo.toml

@@ -12,7 +12,7 @@ anyhow = { workspace = true }
 base64 = { workspace = true }
 chrono = { workspace = true }
 codex-app-server-protocol = { workspace = true }
-codex-core = { workspace = true, features = ["test-support"] }
+codex-core = { workspace = true }
 codex-protocol = { workspace = true }
 codex-utils-cargo-bin = { workspace = true }
 serde = { workspace = true }