fix: blueprints validate exits non-zero on invalid blueprint in non-interactive mode

[kawacukennedy]

What

render blueprints validate now exits non-zero when a Blueprint has validation errors in non-interactive modes (--output json/yaml/text). It also no longer panics if the API returns a 200 with an empty/unparseable body.

Why

This is a validate command — its primary use case is scripting and CI, where the exit code is the contract. Today:

• Interactive mode returns "<file> has validation errors" and exits 1 (cmd/blueprintvalidate.go:201).
• Non-interactive modes print the validation result and return nil, so render blueprints validate -o json ./bad.yaml exits 0 even when the blueprint is invalid. A CI job or script cannot tell valid from invalid.

The two paths disagree on the same command's contract. This change makes non-interactive mode match interactive mode.

How

• After printing the result via command.PrintData, return the same "<path> has validation errors" error when result.Valid is false.
• Guard against a 200 response with an empty body (result == nil), which previously panicked on result.Valid.

Testing

• New tests in cmd/blueprintvalidate_test.go using an httptest server (following the logout_test.go precedent):
  • Valid blueprint succeeds in json/yaml/text modes, and the workspace ID is sent to the endpoint.
  • Invalid blueprint returns an error in all three modes while still printing the validation errors.
  • Invalid blueprint JSON output is valid JSON with "valid": false plus a non-zero exit.
  • Empty 200 response returns a clear error instead of panicking.
• Verified end-to-end against a local mock: invalid blueprint now exits 1.
• go test ./cmd/... ./pkg/... ./internal/... passes.
• No new lint findings (existing lint issues in the repo are pre-existing on main).