PricingOption RootModel doesn't proxy attribute access — breaks intuitive iteration

[bokelley]

Summary

PricingOption is implemented as a RootModel (discriminated union), which means the obvious pattern for iterating a product's pricing options fails:

from adcp.types import Product

product: Product = ...
for po in product.pricing_options:
    print(po.pricing_option_id)  # AttributeError!
    print(po.root.pricing_option_id)  # works, but non-obvious

This is the single biggest DX friction point when building a sales agent with the library.

Steps to Reproduce

from adcp.types import (
    Product, FlatRatePricingOption, FormatId,
    DeliveryMeasurement, DeliveryType, PublisherPropertiesAll,
)

product = Product(
    product_id="test",
    name="Test",
    description="Test product",
    publisher_properties=[
        PublisherPropertiesAll(publisher_domain="example.com", selection_type="all")
    ],
    format_ids=[FormatId(agent_url="http://localhost", id="display_300x250")],
    delivery_type=DeliveryType.guaranteed,
    delivery_measurement=DeliveryMeasurement(provider="Test"),
    pricing_options=[
        FlatRatePricingOption(
            pricing_option_id="flat_1",
            pricing_model="flat_rate",
            fixed_price=100.0,
            currency="USD",
        )
    ],
)

po = product.pricing_options[0]
print(type(po))
# <class 'adcp.types.generated_poc.core.pricing_option.PricingOption'>

print(po.pricing_option_id)
# AttributeError: 'PricingOption' object has no attribute 'pricing_option_id'

print(po.root.pricing_option_id)
# 'flat_1' — works but requires knowing the RootModel internals

Expected Behavior

po.pricing_option_id should work directly. The RootModel wrapper should be transparent to consumers.

Suggested Fix

Add __getattr__ delegation to the PricingOption RootModel:

class PricingOption(RootModel[CpmPricingOption | FlatRatePricingOption | ...]):
    def __getattr__(self, name: str):
        return getattr(self.root, name)

This is backward-compatible — existing .root access still works, but direct attribute access also works.

Relationship to Prior Issues

• GetSignalsRequest as RootModel breaks the library's own inheritance pattern in consumers #138 covered GetSignalsRequest as RootModel breaking inheritance. This is a different symptom of the same pattern: RootModel unions are opaque to consumers.
• Feature Request: Improve Type Ergonomics for Library Consumers #102 covered type ergonomics broadly. This is a specific, actionable instance.

Impact

Anyone building a sales agent needs to iterate pricing options to validate create_media_buy requests. The .root workaround is non-obvious and not documented in any example.

Environment

• adcp: 3.6.0
• Python: 3.12
• Pydantic: 2.x