fix: check uv before model generation

[FanouZeng-TT]

Description

generate_models.sh checks for uv only after cloning UCP, copying schemas, and attempting the first uv run:

rm -rf ucp
git clone ... ucp
...
uv run python preprocess_schemas.py
...
if ! command -v uv &> /dev/null; then

On a machine without uv, the script deletes or replaces the local ucp directory and performs a network clone before failing at preprocessing. The existing installation guidance is therefore reached too late to prevent those side effects.

Fix: move the existing uv prerequisite check next to the git check, before version selection, cloning, or generated-file 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

N/A

Checklist

• I have followed the Contributing Guide (including Conventional Commits title requirements and ! for breaking changes).
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• I have added tests that prove my fix is effective or that my feature works. (An isolated archive with uv removed from PATH exits before replacing a sentinel ucp directory.)
• New and existing unit tests pass locally with my changes.
• (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. (Not applicable; generation output and logic are unchanged.)

Screenshots / Logs (if applicable)

missing-uv regression: exit=1, sentinel=preserved, no clone attempted
unittest: 79 tests passed
pre-commit: 12 hooks passed
git diff --check: passed