fix: preserve array-root request variants

[ShuoRen-TT]

Description

Fix request variant generation for array-root schemas such as totals.json.

_create_single_variant previously injected empty properties and required
members into every generated variant. For an array-root schema,
datamodel-code-generator interpreted the resulting mixed array/object shape as
a union between the intended list and an empty BaseModel. The function also
did not apply nested ucp_request rules under items and its allOf branches.

This change:

• extracts object property filtering into a reusable helper;
• preserves the root shape of array schemas;
• applies request rules to object schemas nested below array items;
• regenerates the create and update totals request models; and
• adds regression coverage for schema preprocessing and generated model
  validation.

The generated totals request aliases are now lists of Total. They no longer
contain a spurious empty model or expose the nested response-only lines
field as a typed request field.

Relationship to #35

This PR and #35 are complementary rather than competing fixes.

• rewrite $ref in top-level oneOf/anyOf branches during variant generation #35 addresses request-variant $ref rewriting for top-level composition
  branches such as oneOf and anyOf.
• This PR addresses array-root shape preservation and nested ucp_request
  filtering under items and allOf.

Both changes touch _create_single_variant, so the PR merged second may need a
small mechanical rebase. This PR intentionally does not include #35's
whole-variant $ref rewrite or its generated fulfillment model changes.

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

Fixes #37

Related: #35

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 2026-04-08.

Validation

• bash generate_models.sh 2026-04-08
• python -m unittest preprocess_schemas_test.py (2 tests passed)
• python -m compileall -q preprocess_schemas.py preprocess_schemas_test.py src/ucp_sdk
• pre-commit on all changed files
• full pre-commit excluding ShellCheck, which requires Docker locally; this PR does not modify shell files