fix(test): prevent panic on missing API/App key

[giannileggio]

Summary

• pup test panicked with slice bounds out of range [:8] with length 0 when DD_APP_KEY (or DD_API_KEY) was not set, because the masking code tried to slice an empty string
• Added explicit empty-key checks before the masking code, returning a clear error message

Changes

• cmd/root.go:208-214 - validate APIKey and AppKey are non-empty before slicing
• cmd/root_test.go - added TestTestCmd_MissingKeys (3 cases) and TestTestCmd_ValidKeys

Testing

• go test ./... all passing
• Manually verified pup test with missing app key now returns a helpful error instead of panicking

[platinummonkey]

fixed in #20

[giannileggio]

fixed in #20

closing this

[github-actions]

📊 Test Coverage Report

Overall Coverage: 80.4%

Threshold: 80% ✅

Coverage by Package

## Coverage by Package

- github.com/DataDog/pup/pkg/auth/callback/server.go:40: 81.2%
- github.com/DataDog/pup/pkg/auth/dcr/client.go:28: 100.0%
- github.com/DataDog/pup/pkg/auth/dcr/types.go:24: 100.0%
- github.com/DataDog/pup/pkg/auth/oauth/client.go:22: 100.0%
- github.com/DataDog/pup/pkg/auth/oauth/pkce.go:24: 85.7%
- github.com/DataDog/pup/pkg/auth/storage/factory.go:53: 94.7%
- github.com/DataDog/pup/pkg/auth/storage/keychain.go:44: 42.9%
- github.com/DataDog/pup/pkg/auth/storage/storage.go:58: 71.4%
- github.com/DataDog/pup/pkg/auth/types/types.go:23: 100.0%
- github.com/DataDog/pup/pkg/client/client.go:32: 94.4%
- github.com/DataDog/pup/pkg/config/config.go:22: 100.0%
- github.com/DataDog/pup/pkg/formatter/formatter.go:31: 100.0%
- github.com/DataDog/pup/pkg/useragent/useragent.go:32: 100.0%
- github.com/DataDog/pup/pkg/util/time.go:20: 95.8%

## Summary

total:								(statements)		80.4%

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📈 Coverage Status: ✅ PASSED - Coverage meets minimum threshold

_{Updated for commit 8d510b9}