fix: use configured discount code in totals test

[FanouZeng-TT]

Description

totals_test.py defined a module-level _DISCOUNT_CODE and used it when driving the discounted checkout helper:

# A percentage discount code recognized by the reference data set. If the
# server under test does not recognize it, the discount-sign test skips.
_DISCOUNT_CODE = "10OFF"
...
updated = self.update_checkout_session(
  checkout_obj, discounts={"codes": [_DISCOUNT_CODE]}
)

The rest of the conformance suite reads the test discount code through
fixture_ctx.get_test_discount_code(), which comes from valid_discount_code in
the configured fixture data with the existing fallback chain. The hardcoded
"10OFF" only matches the default fixture. A server tested with a non-default
fixture, for example "PROMO20", would receive an unrecognized discount code,
return no discount totals entry, and make test_discount_entry_is_negative
skip instead of checking the negative-sign invariant.

Fix: use self.fixture_ctx.get_test_discount_code() in
_discounted_checkout_totals() so the totals sign test applies the same discount
code as the active fixture.

Category (Required)

• Core Protocol: Changes to core UCP JSON schemas, protocol behavior, or specification semantics. (Requires Technical Council approval)
• Governance/Contributing: Changes to contribution process, governance, or project policy. (Requires Governance Council approval)
• Capability: Additions or changes to optional protocol capabilities. (Requires Maintainer approval)
• Documentation: Documentation-only changes. (Requires Maintainer approval)
• Infrastructure: CI, build, release, or repository infrastructure changes. (Requires DevOps Maintainer approval)
• Maintenance: Dependency updates, cleanup, or non-functional maintenance. (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. (Requires Maintainer approval)
• Community Health (.github): Organization health files, templates, or community metadata. (Requires DevOps Maintainer approval)

Related Issues

N/A

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

• I have followed the Contributing Guide for 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 updated the documentation (if applicable).
• My changes generate no new warnings.
• 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.
• Any dependent changes have been merged and published in downstream modules.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk.

Screenshots / Logs (if applicable)

N/A — test-only conformance fix.