[TypeDeclaration] Skip string param inference in ScalarParamTypeByMethodCallTypeRector to avoid unsafe scalar coercion

[TomasVotruba]

ScalarParamTypeByMethodCallTypeRector inferred a param type by copying the declared param type of the called method (CallerParamMatcher::matchCallParamType returns the callee param type node verbatim). For a string callee param this is unsafe: PHP scalar coercion lets a caller legitimately pass int/float/bool.

Real case: a method passes $value to a vendor set(string $key, string $value). The rule added string $value, but the caller actually passes an int and relied on coercion. Under declare(strict_types=1) this then breaks callers with a TypeError.

There is no local signal to tell the safe case from the unsafe one - both are an untyped variable passed to a string param - so the rule now bails out when the inferred type is a plain string. Non-string scalars and unions (int, bool, float, int|bool, bool|string|null) still get inferred.

• guard isMatchingParamType() against pure string
• repoint transform fixtures to an int callee, swap the code sample to int
• add skip_string_caller_param fixture covering the coercion case