feat: add comprehensive PII detection test suite (vllm-project#334)

Add 06-pii-detection-test.py to test PII detection across multiple pathways:

- Batch API PII Detection via unified classifier
- Direct PII API endpoint testing
- ExtProc PII filtering in production pipeline
- Multiple PII types pattern analysis

The test suite includes cache-busting mechanisms using timestamps and UUIDs
to avoid cached responses, comprehensive error reporting, and detailed
documentation of system behavior for regression testing.

Signed-off-by: Yossi Ovadia <[email protected]>
Signed-off-by: liuhy <[email protected]>

Author: yossiovadia