[11/44] Enable TS strict mode in utils-testing

Description

Part of the strict-mode rollout epic #35932. Enable TypeScript strict mode for the utils-testing project using typescript-strict-plugin.

• Dependency layer: 2 (0 = leaf library, 10 = top-level app)
• Internal dependents: 18 project(s)
• Rollout order: 11 / 44
• Note: Test utilities - improves typing of specs across the repo.

All internal dependencies of utils-testing should already be in strict mode before starting (they appear earlier in the rollout order).

Acceptance Criteria

• utils-testing's src is added to the typescript-strict-plugin strict paths config.
• All resulting type errors are resolved (no new any; use explicit types).
• // @ts-strict-ignore is used only where unavoidable, each with a follow-up TODO.
• yarn nx run utils-testing:lint passes (and :build / :test where applicable).
• npx tsc-strict passes for utils-testing.
• yarn nx affected -t build does not break dependents.

Priority

Medium

Additional Context

One project in the bottom-up rollout tracked by epic #35932. Depends on the bootstrap task being merged and on lower-layer projects already being strict.

Start with the strict-mode rollout epic #35932 and the utils-testing project configuration, then add utils-testing/src to the typescript-strict-plugin strict paths. Run npx tsc-strict and resolve the resulting errors with explicit types, using // @ts-strict-ignore only with follow-up TODOs. Verify with the project's lint, build, test, and affected-build commands.