proto: fix debug bundle label selector validation patterns (#1820)

* proto: fix debug bundle label selector validation patterns

Updates LabelSelector validation in debug_bundle.proto to properly support
Kubernetes label constraints including DNS subdomain prefixes with forward
slashes (e.g., app.kubernetes.io/name=redpanda-sandbox).

Changes:
- Label key pattern supports prefix/name format with RFC 1123 validation
- Label key max length increased to 317 chars (253 prefix + 1 slash + 63 name)
- Label value pattern follows Kubernetes 63-character limit
- Both patterns require alphanumeric start/end characters
- Allows hyphens, underscores, and dots in middle positions

Fixes validation failures when using standard Kubernetes labels in rpk debug
bundle commands via Console UI.

* backend: add comprehensive debug bundle validation tests

- Test label selector validation patterns including Kubernetes labels
- Test SCRAM authentication validation constraints
- Test all field validation rules and edge cases
- Ensure proto validation works correctly for debug bundle service

* backend: fix debug bundle validation test formatting

Author: weeco