feat: move schema generation before Docker build, coglet loads from file
Schema generation now runs before the Docker build starts, failing fast
on schema errors before any container work begins.
Build side (pkg/image/build.go):
- Schema generation block moved above Docker image build
- .cog/openapi_schema.json is written before the build context is created
- No functional change to schema gen itself (still uses schemagen.Generate)
Coglet side (crates/coglet/src/worker.rs):
- Loads schema from .cog/openapi_schema.json instead of calling Python
cog._schemas.to_json_schema() via PyO3 at runtime
- Missing schema file: clear warning, predictor accepts any input
- Corrupt schema file: clear warning, predictor accepts any input
- Images built with older cog versions (no schema file) continue to work