F08-T10: Header Integration Tests

[leocamello]

Task

Add integration tests verifying X-Nexus-Fallback-Model header in HTTP responses.

Acceptance Criteria

• Test api_response_includes_fallback_header passes
• Test response_no_fallback_header_when_primary_used passes
• Test streaming_response_includes_fallback_header passes
• Integration tests verify header in HTTP response

File

tests/api_integration.rs or tests/routing_integration.rs

Dependencies

• F08-T07: RoutingResult Struct #95 (T07: RoutingResult struct)
• F08-T08: X-Nexus-Fallback-Model Header #96 (T08: Header implementation)

TDD Requirements

Tests must be written and fail BEFORE implementation.