Make exposed downgrade rules idempotent on repeated application

[TomasVotruba]

A few downgrade rules were not idempotent - running the rule more than once over the same file kept transforming its own output (infinite re-wrapping or re-appending). This is harmless today, but the upcoming rector-src node-traverser simplification drops the created-by-rule reprocessing guard, so each rule must settle on its own.

Fixes:

• DowngradeFlexibleHeredocSyntaxRector - left a marker on the doc indentation attribute once flattened, so a later pass skips the node instead of appending another newline.
• DowngradeNewMethodCallWithoutParenthesesRector - the already-parenthesized guard checked the method-call start token, but the opening paren is prepended to the new var token; now both positions are checked.
• DowngradeIsEnumRector - skip an isEnum() call already wrapped in a method_exists(...) ? ... : false ternary, using the same IS_IF_TERNARY idiom as DowngradeReflectionGetAttributesRector.
• DowngradePhp73JsonConstRector - returning an array replaces the Expression, so the traverser never descended into the ConstFetch to clean the flag; the flag is now cleaned in place, so the error check is appended once.
• DowngradeCovariantReturnTypeRector - only downgrade to a wider ancestor type; a multi-level hierarchy previously flipped the return type back and forth.
• DowngradeProcOpenArrayCommandArgRector - skip a command arg already wrapped in the is_array(...) ? implode(...) : ... ternary (its type is string|array, so the string check did not catch it).

All affected rule tests pass, plus ECS and PHPStan.