ci(asm): fix fastapi tests [backport 2.15] (#11585)

Backport 17631cf from #11580 to 2.15.

httpx 0.28.0 is breaking our tests. This PR pins httpx version for
fastapi threat tests.

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

Co-authored-by: Christophe Papazian <114495376+christophe-papazian@users.noreply.github.com>

hatch.toml

@@ -298,7 +298,7 @@ dependencies = [
     "pytest-cov",
     "requests",
     "hypothesis",
-    "httpx",
+    "httpx<0.28.0",
     "anyio{matrix:anyio:}",
     "fastapi{matrix:fastapi}"
 ]