Add conformance testing to string.format implementation (#307)

Fixes #257 

This adds conformance testing to our implementation of `string.format`.
cel-python does not currently provide a `string.format` function so we
implement our own and add it to the protovalidate-python CEL runtime.

While formatting is not (yet) a part of the cel-spec, they do provide
conformance test data for validating behavior. So, to ensure we are
conformant, we use this test data and run it against our implementation.

See the test data
[here](https://github.com/google/cel-spec/blob/master/tests/simple/testdata/string_ext.textproto).
Note that this file contains various sections for testing. However, we
are only concerned with the `format` and `format_errors` sections. In
addition, since we are only testing for formatting, we do not handle
every `oneof` case or variation in the `SimpleTest` definition -- only
expected results or evaluation errors. If others are added down the road
in future versions of the spec, we can adjust these tests.

Author: smaye81

Makefile

@@ -18,6 +18,10 @@ ADD_LICENSE_HEADER := $(BIN)/license-header \
 		--year-range "2023-2025"
 # This version should be kept in sync with the version in buf.yaml
 PROTOVALIDATE_VERSION ?= v0.11.0
+# Version of the cel-spec that this implementation is conformant with
+# This should be kept in sync with the version in format_test.py
+CEL_SPEC_VERSION ?= v0.24.0
+TESTDATA_FILE := tests/testdata/string_ext_$(CEL_SPEC_VERSION).textproto
 
 .PHONY: help
 help: ## Describe useful make targets
@@ -36,6 +40,7 @@ generate: $(BIN)/buf $(BIN)/license-header ## Regenerate code and license header
 	rm -rf gen
 	$(BIN)/buf generate buf.build/bufbuild/protovalidate:$(PROTOVALIDATE_VERSION)
 	$(BIN)/buf generate buf.build/bufbuild/protovalidate-testing:$(PROTOVALIDATE_VERSION)
+	$(BIN)/buf generate buf.build/google/cel-spec:$(CEL_SPEC_VERSION) --exclude-path cel/expr/conformance/proto2 --exclude-path cel/expr/conformance/proto3
 	$(BIN)/buf generate
 	$(ADD_LICENSE_HEADER)
 
@@ -46,7 +51,7 @@ format: install $(BIN)/license-header ## Format code
 	pipenv run ruff check --fix protovalidate tests
 
 .PHONY: test
-test: generate install ## Run unit tests
+test: generate install gettestdata ## Run unit tests
 	pipenv run pytest
 
 .PHONY: conformance
@@ -69,6 +74,13 @@ checkgenerate: generate
 	@# Used in CI to verify that `make generate` doesn't produce a diff.
 	test -z "$$(git status --porcelain | tee /dev/stderr)"
 
+.PHONY: gettestdata
+gettestdata: $(TESTDATA_FILE)
+
+$(TESTDATA_FILE):
+	mkdir -p $(dir @)
+	curl -fsSL -o $@ https://raw.githubusercontent.com/google/cel-spec/refs/tags/$(CEL_SPEC_VERSION)/tests/simple/testdata/string_ext.textproto
+
 $(BIN):
 	@mkdir -p $(BIN)
 

Pipfile

@@ -9,6 +9,7 @@ protobuf = "==6.*"
 
 [dev-packages]
 pytest = "*"
+pytest-subtests = "*"
 mypy = "*"
 ruff = "*"
 types-protobuf = "==6.30.2.20250503"

Pipfile.lock

@@ -2,8 +2,8 @@
 version: v2
 deps:
   - name: buf.build/bufbuild/protovalidate
-    commit: 0409229c37804d6187ee0806eb4eebce
-    digest: b5:795db9d3a6e066dc61d99ac651fa7f136171869abe2211ca272dd84aada7bc4583b9508249fa5b61300a5b1fe8b6dbf6edbc088aa0345d1ccb9fff705e3d48e9
+    commit: 7712fb530c574b95bc1d57c0877543c3
+    digest: b5:b3e9c9428384357e3b73e4d5a4614328b0a4b1595b10163bbe9483fa16204749274c41797bd49b0d716479c855aa35c1172a94f471fa120ba8369637fd138829
   - name: buf.build/bufbuild/protovalidate-testing
-    commit: 5acbe1f3c8f24ced9466b9ccccad4cb0
-    digest: b5:5e9d54d19ce3d9d368f4b1b5ee4f20094d1c33d0f2dca19536339335c2e70d5ffedbd4fa28e290b59ecae0671c9d2dc20b6b8ebba5a9ac76cbf5f9d2af655ef4
+    commit: 485aa672244b4befa63053fdc75d4a03
+    digest: b5:82930441f8c3ad186d825d226536baec007b94d23fa974b9905abea7b891a43bd276969d7e8a27453b632cb8b1557079decdd3044306a81de34316d2707089be