Improvement: Real Variant Detection via ECU DID Reads

[arvindr19]

Description

Currently, ServiceResolver::activate_base_variant() uses a dummy response to initialize variant detection in the in proxy-core/src/service_resolver.rs.

Context

The CDA's diagnostic engine requires a variant to be selected before service lookup works correctly. The current implementation:

1. Creates a synthetic dummy DiagServiceResponseStruct with empty data
2. Calls manager.detect_variant() to initialize state charts
3. Falls back gracefully if state chart init fails

Additional Context

Related file: proxy-core/src/service_resolver.rs
Related PR: #11