refac(axum-extra): improve test invariants for protobuf.rs extractor

[darth-raijin]

Motivation

Some of the existing tests for the Protobuf extractor can benefit from using available input values directly, and from adding a couple of small assertions to make the tested behavior a bit more explicit.

This PR aims to strengthen the existing tests slightly, without changing any
behavior or structure. It’s a small polish to make the intent of the tests a
little clearer.

Solution

• Where possible, we utilize the value in the input struct for asserting on the response body
• Where missing, we also assert on the expected HTTP response status code
• In prost_decode_error, verify that the rejection body contains the expected prefix from the ProtobufDecodeError definition.
• Replace the hardcoded "content-type" string with the CONTENT_TYPE constant.

[yanns]

thank you!