fix: expose GetSignalsRequest as union alias, not RootModel

[bokelley]

Closes #138

Problem

GetSignalsRequest is generated as RootModel[GetSignalsRequest1 | GetSignalsRequest2]. Pydantic hard-rejects model_config['extra'] on any RootModel subclass at class construction time, breaking the documented pattern of subclassing library types with env-based config:

# This fails with PydanticUserError in 3.6.0+
class MyGetSignalsRequest(LibraryGetSignalsRequest):
    model_config = ConfigDict(extra="forbid")

Fix

Add a plain union alias in aliases.py that shadows the RootModel wrapper in the public API:

# aliases.py
GetSignalsRequest = GetSignalsRequest1 | GetSignalsRequest2

__init__.py now exports this alias instead of the generated RootModel. Consumers can subclass either variant:

class MyGetSignalsRequest(GetSignalsRequest1):
    model_config = ConfigDict(extra="forbid")

Test plan

• GetSignalsRequest is a types.UnionType, not a RootModel subclass
• Union contains exactly GetSignalsRequest1 | GetSignalsRequest2
• Subclassing either variant with custom model_config works without error
• 654 tests pass

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