fix: gateway-sign auto-redacted responses

[hanakannzashi]

Summary

• use a Gateway signature when auto-redaction changes an attested chat-completion response
• suppress the provider-signature fetch for that response
• cover non-streaming and continuous-usage streaming with byte-exact regression tests

#892 was reported for streams. The same auto-redact response rewrite exists for non-streaming completions, so this PR fixes both paths.

Testing

• cargo test -p api --lib
• cargo test -p services 'completions::' --lib
• cargo test -p api --test e2e_all --no-run
• Focused E2E execution is blocked locally by PostgreSQL authentication during test bootstrap.

Fixes #892

[claude]

Review: fix: gateway-sign auto-redacted streams

Targeted, well-scoped fix, and the core reasoning is right: auto-redact disables the event.raw_passthrough branch (completions.rs:1677-1686) and re-serializes every chunk, so a provider TEE signature over the pre-redaction bytes can never match what the client received. Moving the gateway_signature_enabled computation below maybe_redact so it can observe auto_redact_enabled is the correct placement — nothing between the old and new assignment points reads skip_provider_chat_signature, and the field is only consumed later in create_chat_completion_stream (services/src/completions/mod.rs:1676).

Gating on auto_redact_enabled (requested && !redaction_map.is_empty()) rather than auto_redact_requested is also correct and consistent with the existing passthrough guard — a no-PII request keeps byte-exact provider signing.

Note: I couldn't load the existing inline review threads for this PR, so apologies if any of the below was already covered.

⚠️ model_attestation_supported == None still stores a mismatching provider signature

crates/api/src/routes/completions.rs:468-474

auto_redact_enabled && model_attestation_supported.unwrap_or(false)

The unwrap_or(false) mirrors the fallback policy in chat_stream_usage_mode, but that policy doesn't transfer. There, None disables the rewrite itself — bytes stay byte-exact, so keeping the provider signature is sound. Auto-redact rewrites the chunks unconditionally; None does not turn that off.

Failure path: get_models_with_pricing() errors (completions.rs:1451-1462 — warn and continue), so model_attestation_supported = None for a model that is attested. Then:

• gateway_signature_enabled = false → skip_provider_chat_signature = false
• the service layer reads model.attestation_supported == true directly and runs the provider-signature fetch (services/src/completions/mod.rs:141-145, 1676)
• the client still receives re-serialized, un-redacted bytes

Net: GET /v1/signature/{chat_id} returns a provider_tee signature whose response_hash covers bytes the client never saw. That is worse than the pre-fix state in one respect — a confidently-wrong signature rather than a missing one, with nothing in the response telling the client to distrust it. The new unit test at line 3241 currently pins this behaviour (assert!(!auto_redact_requires_gateway_signature(true, None))).

Suggested fix — fail closed when attestation status is unknown but we know we have mutated the bytes:

fn auto_redact_requires_gateway_signature(
    auto_redact_enabled: bool,
    model_attestation_supported: Option<bool>,
) -> bool {
    // Unlike the usage-rewrite modes, `None` cannot fall back to byte-exact
    // passthrough: auto-redact re-serializes the stream regardless. Suppress
    // the provider fetch rather than store a signature over bytes the client
    // never received.
    auto_redact_enabled && model_attestation_supported.unwrap_or(true)
}

Worth confirming store_chat_signature_and_unpin is a safe no-op for a chat_id that was never pinned (non-attested model) before taking unwrap_or(true). If it isn't, the alternative is to keep the gateway-sign decision as-is but set skip_provider_chat_signature = true whenever auto_redact_enabled — no signature at all is the honest answer here.

Notes (non-blocking)

• Non-streaming attested + auto-redact has the same shape. completions.rs:2113 re-serializes the un-redacted body while services/src/completions/mod.rs:1834-1841 unconditionally spawns store_chat_signature_from_provider on model.attestation_supported — skip_provider_chat_signature is not consulted on the non-streaming path at all. The comment at completions.rs:2110 documents dropping the signed payload as deliberate, so I am reading it as accepted scope; flagging only because the stored signature is still served over /v1/signature/{chat_id} and will mismatch there too, in case Auto-redacted streams store a provider signature that cannot match client-received bytes #892 was meant to cover it.
• E2EE + auto-redact is unguarded on the chat plane. /v1/completions rejects both combinations explicitly (completions.rs:3468-3488) and native Anthropic rejects E2EE outright (anthropic.rs:771-782), but maybe_redact never inspects the encryption headers. It stays inert in practice — the detector finds no PII in ciphertext, so redaction_map is empty — but if it ever minted a placeholder, opaque E2EE chunks would get re-serialized. This PR makes that corner better (the signature would at least match the delivered bytes), so no action needed here; it is pre-existing and worth a separate guard.
• Test coverage narrowed. auto_redact_streaming_splits_placeholder_across_chunks was repurposed rather than added alongside, so the default-stream (no stream_options) content-delta split case lost its dedicated test. auto_redact_unredacts_tool_call_arguments_streaming (line 552) still exercises default-stream tail reassembly, so it is not a hole — but a second test rather than a rename would have kept both.
• The switch to .content_type() + .bytes(Bytes::from(request_json.clone())) is a good call for a byte-exact hash assertion, and bytes is already a regular dependency of crates/api, so it is available to the integration test.
• Signature text format matches store_gateway_signature ({request_hash}:{response_hash}), and the tail awaits the store before flushing [DONE] (completions.rs:1960-1998), so the test's fetch-after-stream cannot race.

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