[refactor] Replace parseOSRelease env var hack with mockable function variable

[karthikvetrivel]

Resolves TODO to mock parseOSRelease instead of using environment variable check.

Design Decision:
I used function variable pattern (vs interface) for simplicity. This matches the stdlib approach (e.g., time.Now) and keeps changes minimal.

Testing:
All existing tests pass with mock enabled. No production code behavior changed.

Note: I also added defer f.Close() to prevent file descriptor leaks

[karthikvetrivel]

@cdesiniotis I added this snippet:

originalPath := osReleaseFilePath
defer func() { osReleaseFilePath = originalPath }()

to make sure that osReleaseFilePath gets restored to its original value after the test completes. Now, this only matters if we have later tests that use it as well (which may be unlikely). Let me know if we should keep it.