fix: prevent silent ancestor resolution from malformed YAML paths

[willbuckner]

Fixes #5979

When malformed YAML creates resource paths like "../../base - ../../shared/prod", FileLoader.New() was silently resolving these to existing ancestor directories through path normalization instead of failing appropriately. This could cause resources to be loaded from unintended locations, which can be especially dangerous in CI/CD environments.

Added validation in FileLoader.New() to check directory existence before allowing path normalization, preventing the silent "ancestor snapping" behavior.

Before:

=== RUN   TestNewLoaderMalformedPath
    fileloader_test.go:223:
        	Error Trace:	/Users/wbuckner/dev/kustomize/api/internal/loader/fileloader_test.go:223
        	Error:      	"must build at directory: not a valid directory: '/shared/prod' doesn't exist" does not contain "does not exist"
        	Test:       	TestNewLoaderMalformedPath
--- FAIL: TestNewLoaderMalformedPath (0.00s)
FAIL
FAIL	sigs.k8s.io/kustomize/api/internal/loader	0.757s
FAIL

After:

=== RUN   TestNewLoaderMalformedPath
--- PASS: TestNewLoaderMalformedPath (0.00s)
PASS
ok  	sigs.k8s.io/kustomize/api/internal/loader	(cached)

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• :white_check_mark: login: willbuckner / name: Will Buckner (0767f9018c704ffc97c333b1c00aba0482d193da)

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[koba1t]

/ok-to-test

[koba1t]

Hi @willbuckner I think this PR is worth introducing.

Please fix tests that happen on CI.