fix(server): fix item creation with UCP 01-23 schema

[cusell-google]

Description

Remove some unused fields to fix the conformance test.

Category (Required)

Please select one or more categories that apply to this change.

• 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

Checklist

• I have followed the Contributing Guide.
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• (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.

Screenshots / Logs (if applicable)

Conformance Testing Verification
The updated mock merchant server (built to 2026-01-23 specifications, without referencing deprecated item metadata) successfully processed the entire Universal-Commerce-Protocol/conformance integration suite.

Result: 13 / 13 Test Suites Passed (59 individual assertions)

• ✅ ap2_test.py (test_ap2_mandate_completion)
• ✅ binding_test.py (test_token_binding_completion)
• ✅ business_logic_test.py (8 assertions: e.g., discounts, totals calculations)
• ✅ card_credential_test.py (test_card_credential_payment)
• ✅ checkout_lifecycle_test.py (11 assertions: creation, update, cancel, idempotency)
• ✅ fulfillment_test.py (11 assertions: known/unknown customers, dynamic fulfillments)
• ✅ idempotency_test.py (4 assertions: idempotency keys on creates/updates/cancels)
• ✅ invalid_input_test.py (3 assertions: missing discounts, malformed payloads)
• ✅ order_test.py (4 assertions: fulfillment retrieval, adjustments, order updates)
• ✅ protocol_test.py (2 assertions: schema validation, version negotiation via headers) test_discovery_urls skipped intentionally
• ✅ simulation_url_security_test.py (3 assertions: unauthorized webhook invocations blocked)
• ✅ validation_test.py (6 assertions: out of stock, payment failures, structured errors)
• ✅ webhook_test.py (3 assertions: order addresses webhook propagation)

No HTTP 500 or 422 errors were encountered during the happy path checkout. The models conform completely.

[cusell-google]

Made some updates to also resolve #86 and #87 in the PR.