R: Catch json parse errors

[sverhoeven]

JSON parser in reqres is more picky causing

══ Failed tests ════════════════════════════════════════════════════════════════
── Error ('test-route.R:370:3'): /get_value_at_indices, given string ───────────
<reqres_problem/rlang_error/error/condition>
Error in `request$parse(json = reqres::parse_json())`: Error parsing the request body
Caused by error:
! lexical error: invalid string in json text.
                                       foobar 
                     (right here) ------^
Backtrace:
    ▆
 1. └─route$dispatch(req) at test-route.R:370:3
 2.   └─private$dispatch0(...)
 3.     ├─routr:::with_route_ospan(...)
 4.     └─remotebmi (local) handler(...)
 5.       └─request$parse(json = reqres::parse_json())
 6.         └─reqres::abort_status(...)
 7.           └─rlang::cnd_signal(err)

[ FAIL 1 | WARN 0 | SKIP 0 | PASS 110 ]
Error:
! Test failures.
Execution halted

This PR catches the json parse error and returns them.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.68%. Comparing base (b26146b) to head (9a8b79f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #55       +/-   ##
===========================================
+ Coverage   49.80%   96.68%   +46.87%     
===========================================
  Files           9        2        -7     
  Lines         520      392      -128     
  Branches       27        0       -27     
===========================================
+ Hits          259      379      +120     
+ Misses        258       13      -245     
+ Partials        3        0        -3     

Components	Coverage Δ
Python	∅ <ø> (∅)
Julia	∅ <ø> (∅)
R	96.68% <100.00%> (∅)

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[Copilot]

Pull request overview

This PR updates the R route layer so endpoints that parse JSON request bodies return a structured 400 response when the body is invalid JSON, and adds route tests for malformed JSON inputs.

Changes:

• Added a shared JSON parsing helper that converts parse failures into JSON error responses.
• Applied the helper to POST handlers that parse request bodies.
• Added tests for malformed JSON and JSON-string request bodies.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
R/R/route.R	Adds shared JSON parse error handling and applies it to relevant route handlers.
R/tests/testthat/test-route.R	Adds regression tests for malformed JSON request bodies.

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