Identify and block malformed NO_PROXY values

[HarrisonWAffel]

Issue: rancher/rancher#50041

Problem

The expected format of NO_PROXY is not clearly documented, and some users are under the impression that whitespace can be included in the value. This does not align with the format expected by Go, and also results in subtle issues within the downstream cluster provisioning and upgrade process.

Solution

Update the v1 cluster validator to identify when a malformed NO_PROXY agent environment variable is provided and block create or update operations. The only time that malformed NO_PROXY values are allowed are if a cluster was previously provisioned with a NO_PROXY value which contains spaces.

CheckList

• Test
• Docs

[jakefhyde]

Testing nits

[HarrisonWAffel]

latest commit updates validateHTTPNoProxyVariable to use existing utility functions from the admission package instead of returning raw admissionv1.AdmissionResponse pointers, otherwise core logic is unchanged.