fix(cli): improve assert fake client lookup logic

[FirePheonix]

Explanation

When running the chainsaw assert CLI command using the --resource flag, the internal fake client previously hardcoded resource lookups to always return the very first element in the loaded file (ressources[0]). If a resource file contained multiple manifests, assertions targeting any resource other than the first one would receive the wrong object.

I replaced the hardcoded index access with proper resource filtering by Name, Namespace, and GroupVersionKind, returning a standard NotFound API error if the requested object is missing.

Related issue

There was the following TODO:
pkg/commands/assert/command.go : // TODO: we should improve the lookup logic here

Proposed Changes

• Refactored GetFn in pkg/commands/assert/command.go to match objects by name, namespace, and GVK.
• Refactored ListFn in pkg/commands/assert/command.go to filter items by namespace, GVK, and label selector.
• Updated unit tests in pkg/commands/assert/command_test.go to cover the resourcePath fake client path.

Checklist

• I have read the contributing guidelines.
• I have read the PR documentation guide and followed the process including adding proof manifests to this PR.
• This is a bug fix and I have added unit tests that prove my fix is effective.