fix: check correct variable in subprocess stdin cleanup (#25562)

## Summary

- Fix typo in `onProcessExit` where `existing_stdin_value.isCell()` was
checked instead of `existing_value.isCell()`
- Since `existing_stdin_value` is always `.zero` at that point, the
condition was always false, making the inner block dead code

## The Bug

In `src/bun.js/api/bun/subprocess.zig:593`:

```zig
var existing_stdin_value = jsc.JSValue.zero;  // Line 590 - always .zero
if (this_jsvalue != .zero) {
    if (jsc.Codegen.JSSubprocess.stdinGetCached(this_jsvalue)) |existing_value| {
        if (existing_stdin_value.isCell()) {  // BUG! Should be existing_value
            // This block was DEAD CODE - never executed
```

Compare with the correct pattern used elsewhere:
```zig
// shell/subproc.zig:251-252 (CORRECT)
if (jsc.Codegen.JSSubprocess.stdinGetCached(subprocess.this_jsvalue)) |existing_value| {
    jsc.WebCore.FileSink.JSSink.setDestroyCallback(existing_value, 0);  // Uses existing_value
}
```

## Impact

The dead code prevented:
- Recovery of stdin from cached JS value when `weak_file_sink_stdin_ptr`
is null
- Proper cleanup via `onAttachedProcessExit` on the FileSink  
- `setDestroyCallback` cleanup in `onProcessExit`

Note: The user-visible impact was mitigated by redundant cleanup paths
in `Writable.zig` that also call `setDestroyCallback`.

## Test plan

- Code inspection confirms this is a straightforward typo fix
- Existing subprocess tests continue to pass

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

src/bun.js/api/bun/subprocess.zig

@@ -590,7 +590,7 @@ pub fn onProcessExit(this: *Subprocess, process: *Process, status: bun.spawn.Sta
     var existing_stdin_value = jsc.JSValue.zero;
     if (this_jsvalue != .zero) {
         if (jsc.Codegen.JSSubprocess.stdinGetCached(this_jsvalue)) |existing_value| {
-            if (existing_stdin_value.isCell()) {
+            if (existing_value.isCell()) {
                 if (stdin == null) {
                     // TODO: review this cast
                     stdin = @ptrCast(@alignCast(jsc.WebCore.FileSink.JSSink.fromJS(existing_value)));