chore: replace archived golang/mock with go.uber.org/mock

[ramya18101]

🔧 Changes

• Replaced github.com/golang/mock v1.6.0 with go.uber.org/mock v0.6.0 in go.mod/go.sum.
• Removed tools/tools.go; added a tool go.uber.org/mock/mockgen directive to go.mod.
• Makefile: install the pinned mockgen (dropped @latest).
• Regenerated all 30 mocks under internal/{auth,auth0}/mock (import → go.uber.org/mock/gomock).
• Updated the gomock import across 28 internal/cli/*_test.go files.
• Updated references/testing.md.

No changes to any command, flag, or output.

📚 References

• Archived: https://github.com/golang/mock · Successor: https://github.com/uber-go/mock

🔬 Testing

• make test-mocks — regenerates cleanly and idempotently.
• Unit tests pass: internal/cli, internal/auth0, internal/auth.

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

[developerkunal]

LGTM