Fix generic signatures to collect type params from all enclosing classes

[tanishiking]

Previously, javaSig0 only collected type parameter names from the immediate enclosing class when generating Java generic signatures for methods.

However, this isn't enough with nested classes where outer class type parameters could clash with method-level type parameters.

Now we iterate all enclosing classes to collect their type parameter names, to avoid name conflicts.

Fixes https://github.com/scala/scala3/issues/24619

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Also, in 4afe938, I refactored javaSig0 to collect class-level type parameter names in a Map[String, Name]. This allowed us to eliminate the typeParamSigWithName helper method. Does this somewhat align with your suggestion in https://github.com/scala/scala3/pull/24567#discussion_r2578072745 ? @hamzaremmal