[BugFix] - Provider is added to every response item

[montezdesousa]

1. Why?

   • Before [BugFix] - Serialise response before FastAPI #6279 the symbol was dropped from multiple items responses, to fix that the response is dumped to a dict so that FastAPI validation doesn't drop symbol
   • We also needed provider field in the dumped response so that the discriminated union of outputs can be performed (provider could not be excluded from model_dump)
   • By doing this, provider is added to every response item, but we don't need/want it there

2. What?

   • Refactor discriminated union logic: use callable discriminator instead of field based discriminator. Details https://docs.pydantic.dev/latest/concepts/unions/#discriminated-unions
   • This way we don't need the provider field to discriminated the union, so it's not even a field
   • Centralize discriminator logic inside ProviderInterface::_generate_return_annotations and remove it from SignatureInspector::inject_return_type
   • Some dark magic occurred and the symbol stop being dropped from the response now
   • Removed some methods from OBBject that were used to create docstrings. Added same functionality (refactored) in DocstringGenerator
   • Rebuild the package, it updates the docstrings with new pydocstyle changes

3. Impact:

   • No provider field added to every response results

4. Testing Done:

   • Confirmed balance sheet fields respect provider order