Pin dependency versions and add constraints.txt

Exact versions for direct dependencies, full pip freeze of the tested
image as constraints, both Dockerfiles install with -c constraints.txt.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

requirements-dev.txt

@@ -1,2 +1,2 @@
-pytest
-httpx
+pytest==9.1.1
+httpx==0.28.1