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[dependency] Address review comments on protoc include download

- Map protoc 3.21.x to its release tag (v21.N) when fetching the well-known types.
- Simplify the download to `curl --create-dirs` + `unzip -o` in place.
- `make clean-deps` also removes the downloaded protoc includes.
- Pre-compute the `make mocks` PATH in a `mocks_path` variable.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Yoav Tock <TOCK@il.ibm.com>