test(processor): add unit tests for response.go's ProcessResponse

[kalra-mohit]

Closes #429.

response.go already had decent coverage on RestorePII/BuildRestorer, but ProcessResponse — the function that actually gets called on every response — had none. This adds it.

ProcessResponse's job, stripped down: gate on content type, try to parse the body as JSON (fall back to the original bytes if it doesn't parse), hand off to the provider's RestoreMaskedResponse for the actual PII restoration, then stamp original_response and proxy_metadata onto the result.

What's covered:

• Non-JSON content type passes through byte-for-byte, untouched.
• Malformed JSON falls back to the original body instead of erroring or emitting something truncated.
• The main path, end-to-end: a masked placeholder in choices[0].message.content gets restored to the real value, original_response holds the untouched input bytes, and proxy_metadata gets stamped. original_response is embedded via json.RawMessage, so it comes back as nested JSON rather than a string when you unmarshal the result — took me a second to realize the test needed to account for that instead of comparing it as a plain string.
• The "mapping miss" case from the issue: if a masked-looking token in the response has no matching entry in maskedToOriginal, it's left exactly as-is — not corrupted, not blanked.
• What happens when the provider can't make sense of the response shape (RestoreMaskedResponse errors on something without a choices field): ProcessResponse just logs it and keeps going, and proxy_metadata still gets added. That distinction matters — a caller shouldn't be able to tell "nothing needed restoring" apart from "the proxy gave up silently."
• The proxy-notice wiring: confirms the interception-notice string ProcessResponse builds, and the getAddProxyNotice callback it passes down, actually reach OpenAIProvider and show up in the response content.

Why a real OpenAIProvider instead of a mock: providers.Provider's CreateMaskedRequest and RestoreMaskedResponse are typed against unexported function types declared inside package providers (maskPIIInTextType, restorePIIType, etc.), so nothing defined outside that package can satisfy the interface — a hand-rolled fake from processor_test simply won't compile. I noticed proxy/handler_test.go already deals with this the same way (constructs a real OpenAIProvider, which is cheap since it makes no network calls until you actually send a request), so I followed that existing pattern instead of inventing a different one.

One thing I left deliberately undone: the json.Marshal failure fallback (the branch where re-marshaling the modified response fails and ProcessResponse returns the original body) isn't separately tested. Triggering it needs a provider to write something unmarshalable — a channel, a func — into the decoded response map, and every real provider only ever writes strings. Given the same unexported-interface constraint above, there's no way to force that branch through the public API without a fake, which isn't buildable here. I left a comment explaining that rather than skip it silently.

This repo requires first-time contributors to add themselves to CONTRIBUTORS.md; that entry is in #609 (one of three PRs from the same pass), not duplicated here.

flowchart LR
    A[Incoming response body\n+ contentType + maskedToOriginal] --> B{JSON content type?}
    B -->|no| C[return body unchanged]:::covered
    B -->|yes| D{json.Unmarshal succeeds?}
    D -->|no, malformed| E[return original body]:::covered
    D -->|yes| F["provider.RestoreMaskedResponse\n(e.g. OpenAIProvider)"]
    F -->|error - unrecognized shape| G[log only, keep going]:::covered
    F -->|ok| H["response with PII\nrestored in place\n(unmapped tokens left as-is)"]:::covered
    G --> I[stamp original_response\n+ proxy_metadata]:::covered
    H --> I
    I --> J{json.Marshal succeeds?}
    J -->|yes| K[return modified JSON]:::covered
    J -->|no - not reachable\nvia public API today| L[return original body]:::uncovered

    classDef covered fill:#1f4d2c,stroke:#27ae60,color:#fff
    classDef uncovered stroke-dasharray: 5 5

Loading

Test plan:

• go test ./src/backend/processor/... passes
• go vet and gofmt -l clean
• For the two most consequential assertions — the original_response field and the provider-error-keeps-going behavior — I temporarily removed the field and made the error path return early instead of continuing, confirmed both new tests failed as expected, then reverted

[github-actions]

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[kalra-mohit]

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I have read the CLA Document and I hereby sign the CLA

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[kalra-mohit]

I have read the CLA Document and I hereby sign the CLA

[kalra-mohit]

recheck

[kalra-mohit]

@hanneshapke whenever you get a chance to take a look, happy to address any feedback!