Add unit tests for src/backend/processor/response.go

[hanneshapke]

src/backend/processor/response.go is the only file in the processor package and has no response_test.go. Response post-processing (PII restoration / streaming handling) is on the hot path and worth testing in isolation.

What to add

Create src/backend/processor/response_test.go with table-driven tests for:

• Non-streaming JSON response — masked values are restored to originals
• Streaming/SSE response — chunks are reassembled and restored correctly
• Empty body / no PII — passes through unchanged
• Mapping miss (response contains a masked token with no entry in the map) — fallback is documented and tested

Hints

• Mirror the style of src/backend/pii/database_test.go for table-driven tests.
• Construct mappings manually rather than wiring up the full detector.
• Run go test ./src/backend/processor/....

Difficulty: good first issue, ~1–2 hours.