Fix: properly load and parse NO_PROXY environment variable

[Ndugu2]

What type of PR is this?

/kind bug

What this PR does / why we need it:

This PR fixes the handling of the NO_PROXY environment variable in the Configuration class. Previously:

1. No parsing: The value was stored as a raw string instead of being parsed into a list of domains
2. Precedence issues: When both NO_PROXY and no_proxy were set, the lowercase version could take precedence incorrectly
3. Empty string handling: An empty NO_PROXY="" was treated the same as if the variable wasn't set
4. Default value: self.no_proxy defaulted to None instead of an empty list

These issues caused proxy bypass settings to be ignored, breaking functionality for users relying on no_proxy configurations.

Which issue(s) this PR fixes:

Fixes #2520

Special notes for your reviewer:

The fix implements proper precedence checking using in os.environ rather than os.getenv() to correctly detect when a variable exists even if it's empty. The value is now parsed into a list of domains with whitespace stripped.

Test cases added cover:

• Uppercase NO_PROXY
• Lowercase no_proxy
• Both uppercase and lowercase (precedence test)
• Empty string values
• No environment variable set
• Spaces in comma-separated values

Does this PR introduce a user-facing change?

Fixed: The NO_PROXY environment variable is now properly parsed into a list of domains. Uppercase NO_PROXY correctly takes precedence over lowercase no_proxy. Empty NO_PROXY="" now results in an empty list rather than falling back to the lowercase variant.

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• ✅ login: Ndugu2 / name: Kosea Kalema (e78a303)

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