fix(server): redact transport error details

[ting-hong-shieh]

What

• Return fixed client-visible messages for upstream transport and timeout failures instead of exposing their source errors.
• Preserve the existing HTTP status, error type, and error code for both classifications.
• Add regression coverage for a credential-bearing upstream URL, timeout sanitization, and unchanged non-secret response-decoding details.

Why

client_error previously copied LlmClientError::Transport and LlmClientError::Timeout source strings into HTTP error responses. Reqwest transport errors may include the complete request URL, so credentials stored in base_url query parameters could be returned to callers.

The server now treats transport and timeout sources as internal details at the client-facing error boundary. This is generic and does not depend on a provider or query-parameter name.

Closes #423

How tested

• cargo test -p switchyard-server client_ -- --nocapture
• cargo test -p switchyard-server --test server transport_error_hides_credential_bearing_upstream_url -- --exact --nocapture
• cargo test -p switchyard-server --test server request_and_upstream_errors_use_the_inbound_wire_format -- --exact --nocapture
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test -p switchyard-server
• git diff --check

All tests use local or synthetic errors with CANARY_ADMIN_QUERY_KEY. No provider endpoint or real credential was used.

Notes for reviewers

• Transport failures remain HTTP 502 with upstream_error type and code.
• Timeout failures remain HTTP 504 with upstream_error type and upstream_timeout code.
• InvalidResponse continues to expose its source detail unchanged.
• No new logging was added.

Summary by CodeRabbit

• Bug Fixes
  • Upstream connection failures now display a generic 502 Bad Gateway message without exposing request URLs or credentials.
  • Timeout failures now return a standardized gateway-timeout message.
  • Invalid-response errors continue to provide relevant source details.

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Reviewing files that changed from the base of the PR and between 79f8dd4 and 8cf651f.

📒 Files selected for processing (1)

• crates/switchyard-server/tests/server.rs

🚧 Files skipped from review as they are similar to previous changes (1)

• crates/switchyard-server/tests/server.rs

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Walkthrough

The server now returns fixed messages for upstream transport and timeout errors. Invalid-response errors retain their source details. Tests verify status codes and prevent upstream URLs and credentials from appearing in client-visible errors.

Changes

Upstream error sanitization

Layer / File(s)	Summary
Error mapping and response validation crates/switchyard-server/src/lib.rs, crates/switchyard-server/tests/server.rs	Transport errors return a generic 502 Bad Gateway message. Timeout errors return a fixed gateway-timeout message. Invalid-response errors retain their source detail. Tests verify statuses, messages, and removal of upstream URLs and credentials.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to 8cf65

The change replaces client-visible upstream transport and timeout details with fixed messages while preserving the existing response classifications; no actionable merge-blocking risk remains after normal checks and review.

Poem

A rabbit hops past URLs bright,
And hides the keys from client sight.
Bad gateways speak plain and clear,
While response details still appear.
Timeout bells ring safely now—
Clean error paths beneath the bough. 🐇

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies the server fix that redacts transport error details.
Linked Issues check	✅ Passed	The changes prevent query credentials from appearing in transport errors while preserving error behavior and adding regression coverage for issue #423.
Out of Scope Changes check	✅ Passed	The code and tests directly support error sanitization, timeout handling, and preservation of invalid-response details.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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Actionable comments posted: 1

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Inline comments:
In `@crates/switchyard-server/tests/server.rs`:
- Around line 2024-2025: Add a concise one-line comment immediately before the
transport_error_hides_credential_bearing_upstream_url test attribute,
documenting that transport errors must not expose credentials from the
configured upstream URL.

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Fix all unresolved CodeRabbit comments on this PR:

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• Create a new PR with the fixes

---

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📒 Files selected for processing (2)

• crates/switchyard-server/src/lib.rs
• crates/switchyard-server/tests/server.rs

[ting-hong-shieh]

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