fix(rest/python): stop copying client supplied omit members, safely handle attribution, and return UCP error envelope on validation failures

[damaz91]

Description

The create handlers in both checkout_service and cart_service build response models by unpacking request data via **checkout_data and **cart_data.

1. Omit Field Leakage & Keyword Collisions:
   • In checkout create: continue_url, expires_at, messages, and order were not excluded from checkout_data, allowing client values to echo in the 201 Created response and persist to the session database (Checkout create returns client supplied continue_url, expires_at and messages #190).
   • In cart create: continue_url, expires_at, messages, and links were not excluded from cart_data, allowing client values to echo in the response. Furthermore, passing explicit keyword arguments caused 500 TypeError when these fields were supplied in the request (Cart create trusts request members that cart.json marks ucp_request omit #188).
2. Currency Determination:
   • checkout_service was reading currency from the request via getattr(checkout_req, 'currency', None), allowing clients to override merchant currency (e.g., "currency": "XTS") and crashing on non-string inputs (e.g., "currency": 123) (Checkout create takes currency from the request again, and a wrong typed member returns 500 #189).
   • cart_service was hardcoding "USD" rather than using config.get_default_currency().
3. Line Item ID Server Assignment:
   • checkout_service on direct requests was reading getattr(li, 'id', None), adopting client-supplied line item IDs and crashing on non-string IDs ("id": 123) (Checkout create takes currency from the request again, and a wrong typed member returns 500 #189).
4. Attribution model_dump AttributeError:
   • AttributionCreateRequest is a type alias (dict) in ucp-sdk, so calling source_attribution.model_dump() crashed with AttributeError: 'dict' object has no attribute 'model_dump' on direct checkout creation and cart-to-checkout conversion (Checkout create returns 500 when the request carries attribution #191).
5. Request Validation Error Envelope:
   • RequestValidationError was unhandled, falling back to FastAPI's default flat {"detail": [...]} format instead of the compliant UCP error envelope carrying code, content, and severity (Python validation errors answer with the FastAPI detail shape instead of the UCP envelope #195).

This change:

• Excludes all ucp_request: omit and server-managed fields from create_checkout and create_cart.
• Ensures currency is determined authoritatively via config.get_default_currency() across both checkout and cart create paths.
• Ensures line item IDs on direct create are server-assigned UUIDs, ignoring client-supplied IDs.
• Safely dumps attribution, buyer, context, signals, and discounts via hasattr(obj, "model_dump") check.
• Registers a RequestValidationError handler in server.py returning the standard UCP error envelope on HTTP 422.
• Adds regression unit tests across integration_test.py and cart_test.py covering omit member dropping, currency overriding, line item ID handling, attribution support, and validation error envelopes.

Category (Required)

• Core Protocol: Changes to the base communication layer, global context, or breaking refactors. (Requires Technical Council approval)
• Governance/Contributing: Updates to GOVERNANCE.md, CONTRIBUTING.md, or CODEOWNERS. (Requires Governance Council approval)
• Capability: New schemas (Discovery, Cart, etc.) or extensions. (Requires Maintainer approval)
• Documentation: Updates to README, or documentations regarding schema or capabilities. (Requires Maintainer approval)
• Infrastructure: CI/CD, Linters, or build scripts. (Requires DevOps Maintainer approval)
• Maintenance: Version bumps, lockfile updates, or minor bug fixes. (Requires DevOps Maintainer approval)
• SDK: Language-specific SDK updates and releases. (Requires DevOps Maintainer approval)
• Samples / Conformance: Maintaining samples and the conformance suite. (Requires Maintainer approval)
• UCP Schema: Changes to the ucp-schema tool (resolver, linter, validator). (Requires Maintainer approval)
• Community Health (.github): Updates to templates, workflows, or org-level configs. (Requires DevOps Maintainer approval)

Related Issues

Closes #188
Closes #189
Closes #190
Closes #191
Closes #195

Checklist

• I have followed the Contributing Guide (including Conventional Commits title requirements and ! for breaking changes).
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk.