Fix native generation overwriting empty shared model files

[urbankrepel-povio]

When there are no shared schemas, native generation writes a nearly empty common.models.ts, overwriting checked-in enums that JavaScript generation leaves intact. Omit the native shared model file for an empty schema collection to match JavaScript behavior.

Adds 16 lifecycle regression cases covering JavaScript/native, JSON/YAML, incremental on/off, and models-only output. They assert no shared file is created on fresh output, existing enums and modification times survive regeneration with clearOutput: false, and explicit cleanup removes the stale file. Native cases require full native rendering.

Adds a schema-free /health endpoint to Petstore and derives a health-only Petstore fixture for every existing parity configuration. Both the unit parity suite and CI's Linux/macOS file-list and SHA-256 comparison consume this fixture. Explicit assertions verify that the endpoint is generated, the common model file is omitted, and the canonical native case uses full native rendering. The full Petstore fixture remains covered, including updated endpoint and metadata expectations.

Validation on macOS:

• Reproduced the original failure before the fix.
• Full JavaScript and native suites: 1,053 passed and 13 skipped each.
• CI generation/hash commands: JS and native match across 708 cases, 5,266 generated files, and 5,974 manifest entries each.
• Native build, typecheck, lint, and diff checks passed.

Remote CI must validate Linux/macOS jobs for the latest commit.