build(deps): pin protobuf to 3.20.3 in fuzzer_runner for OSS-Fuzz compatibility (#456)

Add uv constraint-dependencies to prevent protobuf from being updated
beyond 3.20.3, which is required for compatibility with OSS-Fuzz
infrastructure.

This prevents dependabot from creating PRs like #453 that would break
the fuzzer runner.

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: hbrodin

fuzzer_runner/pyproject.toml

@@ -25,6 +25,11 @@ packages = ["src/buttercup"]
 requires = ["hatchling"]
 build-backend = "hatchling.build"
 
+[tool.uv]
+# Must match OSS-Fuzz protobuf version for compatibility
+# See: https://github.com/google/oss-fuzz/blob/master/infra/base-images/base-builder/Dockerfile
+constraint-dependencies = ["protobuf==3.20.3"]
+
 [tool.uv.sources]
 common = { path = "../common", editable = true }