## What this PR does
Add DNS-1035 validation for Application names to prevent creation of
resources with invalid names that would fail Kubernetes resource
creation.
### Changes
- Add `ValidateApplicationName()` function using standard
`IsDNS1035Label` from `k8s.io/apimachinery`
- Call validation in REST API `Create()` method (skipped on `Update` —
names are immutable)
- Add name length validation: `prefix + name` must fit within Helm
release name limit (53 chars)
- Remove rootHost-based length validation — the underlying label limit
issue is tracked in #2002
- Remove `parseRootHostFromSecret` and `cozystack-values` Secret reading
at API server startup
- Add unit tests for both DNS-1035 format and length validation
### DNS-1035 Requirements
- Start with a lowercase letter `[a-z]`
- Contain only lowercase alphanumeric or hyphens `[-a-z0-9]`
- End with an alphanumeric character `[a-z0-9]`
Fixes#1538Closes#2001
### Release note
```release-note
[platform] Add DNS-1035 validation for Application names to prevent invalid tenant names
```
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **New Features**
* Enforced DNS-1035 name format and Helm-compatible length limits for
applications during creation; name constraints surfaced early and
reflected in API validation.
* **Documentation**
* Updated tenant naming rules to DNS-1035, clarified hyphen guidance,
and noted maximum length considerations.
* **Tests**
* Added comprehensive tests covering format and length validation,
including many invalid and boundary cases.
* **API**
* OpenAPI/Swagger schemas updated to include name pattern and max-length
validation.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
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