fix: route invalid_input order requests through the discovered endpoint

[XiaolongZhang-TT]

Description

The four order GET/PUT calls in invalid_input_test.py used a raw "/orders/{order_id}" path against the httpx base_url, while every order call in order_test.py goes through self.get_order_url(order_id), which resolves the shopping-service endpoint discovered from /.well-known/ucp:

def get_order_url(self, order_id: str) -> str:
    return self.get_shopping_url(f"/orders/{order_id}")   # {discovered_endpoint}/orders/{id}

The raw path only works when the shopping service is hosted at the server root. For a conformant server whose shopping service lives at a sub-path, these requests 404 before validation ever runs — so test_invalid_adjustment_status and test_malformed_adjustment_payload would fail (or pass) for the wrong reason (404 instead of the intended 422) instead of exercising the intended validation path.

Switched all four to get_order_url to match the rest of the suite. Behavior is unchanged for the sample server, whose shopping endpoint is at the root.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected, including removal of schema files or fields)
• Documentation update

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Is this a Breaking Change or Removal?

N/A — test-only fix, no schema/field removal.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (N/A — test-only fix)
• My changes generate no new warnings
• 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
• Any dependent changes have been merged and published in downstream modules