fix(security): reject empty credential secrets

[GatewayJ]

Type of Change

• New Feature
• Bug Fix
• Documentation
• Performance Improvement
• Test/CI
• Refactor
• Other:

Related Issues

Fixes rustfs/backlog#1096

Summary of Changes

• reject configured credential Secrets whose data field is absent or empty
• centralize required-key, UTF-8, and minimum-length checks in the Context validation boundary
• preserve the existing stable SNAFU errors and status mapping
• add focused coverage for valid data, absent data, missing keys, invalid UTF-8, and short values

Checklist

• I have read and followed the CONTRIBUTING.md guidelines
• Passed make pre-commit (fmt-check + clippy + test + console-lint + console-fmt-check)
• Added/updated necessary tests
• Documentation: N/A; the existing credential Secret contract already requires both keys
• CHANGELOG: N/A; this enforces the documented validation contract
• CI/CD passed (pending GitHub Actions)

Impact

• Breaking change (CRD/API compatibility)
• Requires doc/config/deployment update
• Other impact: invalid configured credential Secrets now block reconciliation with the existing CredentialSecretMissingKey status

Verification

make pre-commit

Additional Notes

Kubernetes Secret retrieval remains in Context, while credential invariants are evaluated by a private pure helper. The existing byte-length behavior is intentionally preserved to keep this fix scoped to missing Secret data.

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