contrib/echo: appsec: fix status code reporting when blocking users

[Hellzy]

What does this PR do?

Fixes status code reporting when blocking users with echo.
In this specific case, we don't call echo's error handlers since the WAF actions will write the response instead, and calling
the echo handler would short circuit that, leading to incorrect status code reporting

Motivation

This is a bugfix.

Describe how to test/QA your changes

This scenario is tested in the appsec's echo related tests.

Reviewer's Checklist

• If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
• Changed code has unit tests for its functionality.
• If this interacts with the agent in a new way, a system test has been added.

[pr-commenter]

Benchmarks

Comparing candidate commit 22f31b9 in PR branch francois.mazeau/echo-status-code-fix with baseline commit ccb1ff2 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 6 cases.

[Julio-Guerra]

I had to check all of that locally to better understand how gRPC and other HTTP frameworks were working without this check. But I get it now and is a good example of why we will have to export our public error type when we add the HTTP body blocking (to have a single error check for both cases).