feat(generator): support arbitrary ucp refs and regenerate c5c6139 models

[caohy1988]

Regenerates the SDK against Universal-Commerce-Protocol/ucp@c5c61396ebf9a5afb9c4169bfa3af1409daa6fd6 — the identity-linking OAuth 2.0 foundation merge from PR #354 plus the v2026-04-08 features. Also fixes a long-standing preprocessor gap that was producing RootModel[Any] wrappers for any capability extension whose $defs lived under a reverse-domain key (identity_linking, fulfillment, …).

Two commits:

• 3388931 — generator + preprocessor + tests + CI
• 47fac9c — regenerated model tree

What changed since the previous push (CI/review feedback):

• End-of-file normalization in the generator. Pre-commit's end-of-file-fixer was firing on every regen because codegen emits a trailing blank line on each generated module. The script now post-processes the entire tree under src/ucp_sdk/models/schemas and collapses the EOF to exactly one newline. Verified: a second consecutive ./generate_models.sh --ref c5c6139 produces a byte-identical __init__.py, and pre-commit run end-of-file-fixer --all-files reports Passed.

Earlier review-cycle changes still on this branch:

• set -euo pipefail + git -C ucp checkout + git -C ucp rev-parse --verify HEAD. A bad ref aborts before the regen step rather than silently rebuilding from the default branch.
• ruff check --fix --ignore "D,E501" matches .pre-commit-config.yaml so benign auto-generated docstring noise no longer aborts the script under set -e.
• _schema_ref.py is post-formatted with uv run ruff format so the committed file matches whatever ruff would produce on subsequent runs. Long --ref values that ruff would otherwise wrap are pre-wrapped at write time.
• lift_capability_extension_defs() lifts capability-extension $defs from dev.ucp.<…>/{platform,business}_schema to top-level {file_stem}_{platform,business}_schema and adds a root oneOf so codegen produces typed Unions. Iteration uses an explicit tuple ("platform_schema", "business_schema") rather than a set, so diffs are deterministic across Python hash seeds. Verified no external $ref points into the dotted-key path before removing it.
• IdentityLinking exposes typed ScopePolicy, ScopeToken, Config, IdentityLinkingPlatformSchema, IdentityLinkingBusinessSchema, with Config.scopes: dict[ScopeToken, ScopePolicy]. The smoke test instantiates a real business-side payload and asserts the typed walk all the way down.
• tests/test_generate_models.py invokes the generator with a nonexistent SHA in a temp tree and asserts nonzero exit plus that _schema_ref.py wasn't rewritten.

Reproducing locally:

./generate_models.sh --ref c5c61396ebf9a5afb9c4169bfa3af1409daa6fd6
./generate_models.sh --ref c5c61396ebf9a5afb9c4169bfa3af1409daa6fd6   # second run — no diff
uv run pytest -v                      # 11 passed
uv tool run pre-commit run --all-files
shellcheck generate_models.sh         # clean

Acceptance for issue #8: shopping/cart.py, shopping/catalog_{search,lookup}.py (with get_product_* defs), common/identity_linking.py (typed config.scopes / scope_policy), all four Context fields, Total.amount = SignedAmount (negatives accepted), open total / error / info / warning vocabularies, standalone info_code / warning_code, provenance constant.