gcp-auth: handle CredentialsFromJSON error and guard nil creds

[nitishagar]

Problem

enableAddonGCPAuth can panic with a nil-pointer dereference at creds.JSON (pkg/addons/addons_gcpauth.go:101) when:

• google.FindDefaultCredentials fails, AND
• detect.IsCloudShell() is true, AND
• CLOUDSDK_CONFIG is unset, the ADC file is unreadable, or google.CredentialsFromJSON fails

In the original code the CredentialsFromJSON error was silently discarded (creds, _ = ...), leaving creds nil and producing a panic instead of an actionable error message.

Fixes #23012

Changes

pkg/addons/addons_gcpauth.go

• Extract Cloud Shell ADC loading into an unexported credentialsFromCloudShellADC(ctx) helper that returns (*google.Credentials, error), making the failure path unit-testable and the error non-discardable.
• In the Cloud Shell branch: call the helper; on failure log the error via klog.Warningf and fall through with creds nil. No exit here — Cloud Shell runs on a GCE VM where detect.IsOnGCE() is true, so today's successful path (ADC load fails → GCE early-return → nil return with warning) is preserved.
• Add an explicit creds == nil guard after the GCE early-return with an actionable exit.Message (same wording as line 85), making the nil-deref at creds.JSON structurally impossible.

pkg/addons/addons_gcpauth_test.go (new)

Unit tests for credentialsFromCloudShellADC:

• CLOUDSDK_CONFIG unset → error
• env set, file missing → error
• file with invalid JSON → error
• file with valid authorized_user JSON → non-nil creds, creds.JSON equals file contents

Deprecation note

The issue title asks whether golang.org/x/oauth2/google.FindDefaultCredentials and CredentialsFromJSON must be replaced. These functions are in maintenance mode but are not removed or broken. Migration to cloud.google.com/go/auth is a separate, larger change and is out of scope for this bug fix.

Testing

GOPROXY=direct go test -v -run TestCredentialsFromCloudShellADC ./pkg/addons/...

All 4 tests pass. go vet ./pkg/addons/... clean.

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