fix: add logic to check for IAM RPC methods within the service before adding mixin to avoid generating duplicate methods

[shivanee-p]

Adds unit tests and creates baselines for testing code generation

Fixes #7659 🦕

[sofisl]

Ok this is awesome! A few questions/comments generally:

1. We also support an LRO mixin. Could you make sure that's not duplicated as well? https://docs.google.com/document/d/1YAeaLHghuvEJJtcjWFv1OaMZJgkRQG-FSVqlJJN5YgI/edit?resourcekey=0-KMHBem3k-xSFVxqUQ1QV1g&tab=t.0#heading=h.ad847mq3hi3j
2. Do we want to drill down by the actual rpc name? Like, say an API declares a mixin AND directly declares a method. Should we assume that if one rpc exists, we just give precedence to that and exclude the mixin entirely? or we do we do it function by function? Or, maybe we just say more generally, don't produce duplicative function names (and don't do it function by function). I'm not really sure what path we'd want to take, my opinion is that maybe we just say we have no duplicative functions for any mixin if it's declared, without looking specifically function by function name (like in your comparison, I'm wondering what if there's a capitalization issue or a just some slight discrpancy in the hard-coding of the function name). we should just get a list of all functions in the mixin and make sure none of these are also declared in the proto.
3. The actual function of how you are excluding functions in the templates makes sense and looks good! I just think we want to clean up how we're deciding what to add (specifically in proto.ts file)