refactor: Add a shared Apify client test mock #1251

Context

Unit tests contain 32 unsafe Apify client casts across 16 files, usually inside repeated local client stubs.

Scope

• Add mockApifyClient() to tests/unit/helpers/tool_context.ts.
• Centralize the unsafe cast in that helper.
• Accept explicit top-level client method overrides.
• Migrate existing local stubs where the helper preserves their behavior and spies.
• Keep resource-specific behavior visible in each test.

Out of scope

• Production code changes.
• A generic mock that invents resource behavior.

Done when

• Unsafe Apify client casts are centralized in the shared helper.
• Existing unit tests pass.
• Required repository checks pass.

Split from #1065 after verification.