Reinforce existing-infrastructure search before creating new test doubles

[wtmoose]

Friction
The agent created an inline test error type instead of finding an existing equivalent in a shared test module. The infrastructure guide's "identify existing infrastructure" step was not followed rigorously. The human had to point to the existing file.

Suggestion
Add a checklist item or explicit step to the infrastructure phase: "Before creating any new test double, fixture, or helper, search the test module and all shared test modules for existing equivalents. List what was found (or confirm nothing exists) before proposing new infrastructure." This makes the search auditable and prevents duplication.

Affected files
Guides/Test/unit-test-infrastructure-guide.md