Fix the InvocationExpression.findCallable() implementation

[ggleyzer]

This issue was reported by Dima on Discord:

In a presence of multiple methods with exact matching of argument types, but different return types, the compiler chooses a "random" one to call.

Claude's analysis of "collectMatchingMethods()" is quite precise:

Ecstasy deliberately allows return-type overloading (return type is part of the full method signature) — so the class declaration itself is legal by design, unlike Java. But the selection behavior when no return type context is available is broken.

The fExact short-circuit was designed for the case where argument types exactly match a method's parameter types — a meaningful tiebreaker. With zero parameters, fExact is vacuously true
for every candidate simultaneously. The early return fires on whichever method the HashSet happens to serve up first, silently bypassing chooseBest's ambiguity detection entirely.

The correct behavior when there's no return type context to disambiguate between return-type overloads is to report SIGNATURE_AMBIGUOUS — forcing the caller to provide context (e.g., Int
x := b.test()). Instead, the compiler picks one silently based on hash order, which is both non-deterministic across JVM runs (hash randomization) and semantically wrong.