fix: return structured JSON for malformed request bodies

[dymchenkko]

Description

JSON-body endpoints returned Axum's plain-text rejection when a request body
failed to deserialize (empty {}, missing field, invalid token address). Every
other error in this API is structured JSON, so clients that parse all responses
as JSON break. This makes that path consistent.

Reviewer notes: I added a generic Error schema (vs. per-endpoint enums) — open
to switching. Invalid syntax is 400, already documented per endpoint, so I
only documented the structured body on 422.

Changes

• Add a Json<T> extractor (api/extract.rs) wrapping axum::Json that, on a
  JsonRejection, returns the API's error("InvalidJson", …) with the
  rejection's status (422/400). Identical to axum::Json on success.
• Apply it to all JSON-body endpoints: post_quote, post_order,
  cancel_order, put_app_data, debug_simulation (cancel_orders /
  orders/by_uids parse raw bytes, unaffected).
• OpenAPI: add a generic Error schema, referenced from the affected 422s.

How to test

cargo test -p orderbook --lib api::extract — covers {} → 422 JSON (#4439),
invalid field value → 422 JSON (#4440), invalid syntax → 400 JSON.

Manual: POST /api/v1/quote with body {} → now 422 + application/json
{"errorType":"InvalidJson",…} instead of text/plain.

Related Issues

Fixes #4439
Fixes #4440

[github-actions]

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[gemini-code-assist]

Code Review

This pull request introduces a custom Json extractor in crates/orderbook/src/api/extract.rs that wraps Axum's native extractor. When request deserialization fails, it returns a structured JSON error response containing errorType and description instead of plain text. The API endpoints in cancel_order.rs, debug_simulation.rs, post_order.rs, post_quote.rs, and put_app_data.rs have been updated to use this new extractor. Additionally, the OpenAPI specification (openapi.yml) has been updated to document the Error schema for 422 responses. No critical issues were found, and there is no feedback to provide.

[dymchenkko]

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