feat: validate model inputs before build and run
Validate CLI inputs for `cog predict`, `cog run`, and `cog train` against
the model's OpenAPI schema before the expensive build/pull/start steps, so
bad inputs fail fast instead of after a full Docker build or container start.
- Add pkg/predict/input_validation.go with schema-driven validation that
reports friendly errors (unknown/missing inputs, type mismatches, enum
values, numeric constraints).
- Extract static schema generation into pkg/schema/static so both the image
build and CLI preflight share one code path.
- predict/train: generate the schema up front from cog.yaml (local source)
or the image's openapi_schema label (existing image) and validate before
building/starting. Images without the label fall back to the runtime
schema fetched after start, preserving prior behavior.
- Surface file-read failures with the offending input name.
- Add unit and integration tests covering the new validation paths and pin
user-facing error wording to guard against kin-openapi changes.
