JSpecify: improve inference for generic methods based on method reference arguments

[msridhar]

We now integrate inference of types for method references passed to generic method calls as part of the overall inference process for generic method calls. Includes handling of method references to varargs methods (which has a surprising number of corner cases).

Summary by CodeRabbit

• Bug Fixes

  • More precise nullability checks for method references (bound, unbound, static, non-static) with improved qualifier/type substitution and generic-argument handling, reducing incorrect parameter nullness reports in JSpecify mode.
  • Generic method references now participate in nullability inference, reducing false positives for generic method overrides invoked via method references.

• Tests

  • Expanded and renamed tests covering bound/unbound/static method references, generic instance refs, stream/map scenarios, and related inference behaviors.

[lazaroclapp]

Did another pass, minimal nits. I know I need to catch up to the changes that just came in, but this is starting to get to the point of complexity where I'd link issues for the remaining things and open separate PRs, personally 🙂

Edit: Never mind, I think the new changes were mostly the rebase, unless I am missing something.