refactor: make unified-exec zsh-fork state explicit (#14633)

## Why

The unified-exec path was carrying zsh-fork state in a partially
flattened way.

First, the decision about whether zsh-fork was active came from feature
selection in `ToolsConfig`, while the real prerequisites lived in
session state. That left the handler and runtime defending against
partially configured cases later.

Second, once zsh-fork was active, its two runtime-only paths were
threaded through the runtime as separate arguments even though they form
one coherent piece of configuration.

This change keeps unified-exec on a single session-derived source of
truth and bundles the zsh-fork-specific paths into a named config type
so the runtime can pass them around as one unit.

In particular, this PR introduces this enum so the `ZshFork` variant can
carry the appropriate state with it:

```rust
#[derive(Debug, Clone, Eq, PartialEq)]
pub enum UnifiedExecShellMode {
    Direct,
    ZshFork(ZshForkConfig),
}

#[derive(Debug, Clone, Eq, PartialEq)]
pub struct ZshForkConfig {
    pub(crate) shell_zsh_path: AbsolutePathBuf,
    pub(crate) main_execve_wrapper_exe: AbsolutePathBuf,
}
```

This cleanup was done in preparation for
https://github.com/openai/codex/pull/13432.

## What Changed

- Replaced the feature-only `UnifiedExecBackendConfig` split with
`UnifiedExecShellMode` in `codex-rs/core/src/tools/spec.rs`.
- Derived the unified-exec mode from session-backed inputs when building
turn `ToolsConfig`, and preserved that mode across model switches and
review turns.
- Introduced `ZshForkConfig`, which stores the resolved zsh-fork
`AbsolutePathBuf` values for the configured `zsh` binary and `execve`
wrapper.
- Threaded `ZshForkConfig` through unified-exec command construction and
the zsh-fork preparation path so zsh-fork-specific runtime code consumes
a single config object instead of separate path arguments.
- Added focused tests for constructing zsh-fork mode only when session
prerequisites are available, and updated the zsh-fork expectations to be
target-platform aware.

## Testing

- `cargo test -p codex-core zsh_fork --lib`


---
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* #13432
* __->__ #14633

codex-rs/core/src/unified_exec/process_manager.rs

@@ -578,8 +578,10 @@ impl UnifiedExecProcessManager {
             Some(context.session.conversation_id),
         ));
         let mut orchestrator = ToolOrchestrator::new();
-        let mut runtime =
-            UnifiedExecRuntime::new(self, context.turn.tools_config.unified_exec_backend);
+        let mut runtime = UnifiedExecRuntime::new(
+            self,
+            context.turn.tools_config.unified_exec_shell_mode.clone(),
+        );
         let exec_approval_requirement = context
             .session
             .services