Return response body in FeignException from errorReading (#2618)

[seonwooj0810]

Fixes #2618

Problem

FeignException.errorReading(...) builds the exception by passing request.body() and request.headers() into the constructor parameters named responseBody / responseHeaders:

return new FeignException(
    response.status(),
    format(...),
    request,
    cause,
    request.body(),     // <- responseBody slot
    request.headers()); // <- responseHeaders slot

As reported in #2618, this means that when an IOException is thrown while reading/decoding the response (see InvocationContext.decode and ResponseHandler.handleResponse), FeignException.responseBody() / content() / contentUTF8() / responseHeaders() return the request body and headers instead of the response. The maintainer confirmed in the thread that this looks like a bug.

Fix

Read the response body into a byte[] and pass the response headers, mirroring the existing errorStatus(...) method. Reading is wrapped in a try/catch (IOException ignored) so a streamed or already-consumed body degrades gracefully to an empty body rather than throwing (consistent with errorStatus).

Test evidence

Strengthened the existing FeignExceptionTest.canCreateWithRequestAndResponse to assert the content: the exception now carries the response body ("response") rather than the request body ("data"), and exposes the response headers. Before the fix this assertion fails (contentUTF8() returned "data").

./mvnw -pl core test -Dtest=FeignExceptionTest
Tests run: 14, Failures: 0, Errors: 0, Skipped: 0
BUILD SUCCESS

Verification done: (1) no in-flight PR (gh pr list --search "responseBody"/"errorReading" empty); (2) the only self-claim is a 2024-11-03 question that never produced a PR (>21 days stale); (3) code-focused change in FeignException.java + test; (4) confirmed the request.body() pattern still present on current master; (5) maintainer acknowledged the bug in-thread. Commit is DCO signed-off; google-java-format validation passes.