ci(asm): fix fastapi tests (#11580)

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

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## Reviewer Checklist
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(cherry picked from commit 17631cf)

hatch.toml

@@ -273,7 +273,7 @@ dependencies = [
     "pytest-cov",
     "requests",
     "hypothesis",
-    "httpx",
+    "httpx<0.28.0",
     "anyio{matrix:anyio:}",
     "fastapi{matrix:fastapi}"
 ]