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/src/lib.rs

@@ -145,6 +145,8 @@ pub use file_watcher::FileWatcherEvent;
 pub use safety::get_platform_sandbox;
 pub use tools::spec::parse_tool_input_schema;
 pub use turn_metadata::build_turn_metadata_header;
+#[cfg(any(test, feature = "test-support"))]
+pub use unified_exec::set_deterministic_process_ids_for_tests;
 // Re-export the protocol types from the standalone `codex-protocol` crate so existing
 // `codex_core::protocol::...` references continue to work across the workspace.
 pub use codex_protocol::protocol;