test: isolate mock-contaminating tests into zauc-mocks-* directories

Bun's `mock.module()` is process-global. Tests that use it permanently
replace the mocked module for all subsequent tests in the same process.
The `mock.restore()` only restores function mocks (spyOn), not module mocks.

Move the 4 test files causing contamination into isolated subdirectories:
- migrate-legacy-plugin-entry.test.ts (mock.module("node:fs"))
- connected-providers-cache.test.ts (reads cache from filesystem)
- legacy-plugin-warning.test.ts (reads config from filesystem)
- subagent-resolver.test.ts (spies on shared modules)

This forces bun to run each in a separate process, preventing the
process-global mock contamination that's been causing 68 CI failures.