Remove deterministic_process_ids feature to avoid duplicate codex-core builds (#11393)

## Why

`codex-core` enabled `deterministic_process_ids` through a self
dev-dependency.
That forced a second feature-resolved build of the same crate, which
increased
compile time and test latency.

## What Changed

- Removed the `deterministic_process_ids` feature from
`codex-rs/core/Cargo.toml`.
- Removed the self dev-dependency on `codex-core` that enabled that
feature.
- Removed the Bazel `deterministic_process_ids` crate feature for
`codex-core`.
- Added a test-only `AtomicBool` override in unified exec process-id
allocation.
- Added a test-support setter for that override and re-exported it from
`codex-core`.
- Enabled deterministic process IDs in integration tests via
`core_test_support` ctor.

## Behavior

- Production behavior remains random process IDs.
- Unit tests remain deterministic via `cfg(test)`.
- Integration tests remain deterministic via explicit test-support
initialization.

## Validation

- `just fmt`
- `cargo test -p codex-core unified_exec::`
- `cargo test -p codex-core --test all unified_exec -- --test-threads=1`
- `cargo tree -p codex-core -e features` (verified the removed feature
path)

codex-rs/core/Cargo.toml

@@ -114,7 +114,6 @@ wildmatch = { workspace = true }
 zip = { workspace = true }
 
 [features]
-deterministic_process_ids = []
 test-support = []
 
 
@@ -150,9 +149,6 @@ keyring = { workspace = true, features = ["sync-secret-service"] }
 assert_cmd = { workspace = true }
 assert_matches = { workspace = true }
 codex-arg0 = { workspace = true }
-codex-core = { path = ".", default-features = false, features = [
-    "deterministic_process_ids",
-] }
 codex-otel = { workspace = true, features = [
     "disable-default-metrics-exporter",
 ] }