Fix MailableMethodsInBuild destroying constructor formatting

[superbiche]

Summary

• Fix MailableMethodsInBuild formatter collapsing multi-line constructor parameters (e.g. promoted properties) into a single line
• Both constructorVisitor and buildVisitor now mutate $node->stmts in-place instead of creating new ClassMethod nodes, preserving PhpParser's printFormatPreserving identity check
• Early-return null when no statements need moving, so untouched constructors aren't re-printed at all

Example

Given a Mailable with multi-line promoted constructor properties:

public function __construct(
    public readonly ?AlertErrorTypeEnum $type,
    public readonly string $hour,
    public readonly int $number,
    public readonly Carbon $now
) {}

Before (broken) — the formatter collapses everything to one line:

public function __construct(public readonly ?AlertErrorTypeEnum $type, public readonly string $hour, public readonly int $number, public readonly Carbon $now)
{
}

After (fixed) — constructor formatting is preserved as-is.

Test plan

• Existing 3 tests pass (no regression)
• New test: does_not_alter_constructor_parameter_formatting — verifies a Mailable with multi-line promoted constructor properties is not reformatted

[driftingly]

Thanks @superbiche 🙌