Fix: Handle V8 Warnings in DisallowJavascriptExecutionScope

Fixes: #63473

The Problem

When the Node.js REPL runs with preview mode enabled, certain code (such as
asm.js modules) causes V8 to emit deprecation warnings inside a
DisallowJavascriptExecutionScope : a V8 context where JavaScript execution
is temporarily forbidden.

Node's warning handler (PerIsolateMessageListener) was unconditionally
calling ProcessEmitWarningGeneric, which invokes process.emit() -> a JS
call which is inside this forbidden scope, causing a fatal crash:

Fatal error in , line 0

Invoke in DisallowJavascriptExecutionScope

What Changed

Modified PerIsolateMessageListener in src/node_errors.cc to check
can_call_into_js() before attempting to emit warnings.

When JavaScript cannot be called (i.e. we are inside a forbidden scope),
the warning is deferred via env->SetImmediate() until after the scope
exits at which point it is safe to call JavaScript again.

Files changed:

• src/node_errors.cc -> added can_call_into_js() guard with deferred emission
• test/parallel/test-repl-asm-warning-no-crash.js : new regression test

What Is Preserved

The fix does not degrade warning quality in any case:

Approach

Instead of falling back to raw fprintf(stderr, ...) (which loses structured
formatting and --redirect-warnings support), this fix uses env->SetImmediate()
to queue the warning for the next safe event loop iteration.

This is an established Node.js pattern for deferring work that requires
JavaScript execution. The lambda captures the warning string by value,
ensuring it remains valid until the callback fires.

// Before (crashes inside DisallowJavascriptExecutionScope)
ProcessEmitWarningGeneric(env, message, ...);

// After (defers safely when JS is forbidden)
if (!env->can_call_into_js()) {
  std::string deferred_msg = message;
  env->SetImmediate([deferred_msg](Environment* env) {
    ProcessEmitWarningGeneric(env, deferred_msg, ...);
  });
  return;
}
ProcessEmitWarningGeneric(env, message, ...);

How to Test

Run the regression test in isolation:

node test/parallel/test-repl-asm-warning-no-crash.js

Or as part of the test suite:

npm test -- test/parallel/test-repl-asm-warning-no-crash.js

Regression Test

test/parallel/test-repl-asm-warning-no-crash.js reproduces the issue by:

1. Starting a Node.js REPL with usePreview: true (triggers the inspector path)
2. Sending the asm.js reproduction case from V8 warnings crash process within DisallowJavascriptExecutionScope #63473
3. Asserting the process exits cleanly with exit code 0
4. Asserting no FATAL ERROR appears in stderr

Checklist

• Regression test added
• No breaking changes to existing behaviour or public API
• --redirect-warnings routing preserved
• No changes outside the two files listed above
• Commit message follows Node.js format
• No trailing whitespace or formatting changes to unrelated code