Fix: make test-coverage to properly enable coverage for all test suites (#2989)

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# Rationale for this change
The `test-coverage` target was failing with "No data to combine" because
the `COVERAGE=1` variable wasn't being propagated to prerequisite test
targets.

Make's target-specific variable syntax (`test-coverage: COVERAGE=1`)
only sets the variable for that specific target, not for its
prerequisites when they execute as separate Make invocations.

Found this out when verifying release, which is the only place we run
`make test-coverage`. CI runs `make coverage-report`

## Are these changes tested?
Ran `make test-coverage` before and after

## Are there any user-facing changes?

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Author: kevinjqliu

Makefile

@@ -133,8 +133,9 @@ test-gcs: ## Run tests marked with @pytest.mark.gcs
 	sh ./dev/run-gcs-server.sh
 	$(TEST_RUNNER) pytest tests/ -m gcs $(PYTEST_ARGS)
 
-test-coverage: COVERAGE=1
-test-coverage: test test-integration test-s3 test-adls test-gcs coverage-report ## Run all tests with coverage and report
+test-coverage: ## Run all tests with coverage and report
+	$(MAKE) COVERAGE=1 test test-integration test-s3 test-adls test-gcs
+	$(MAKE) coverage-report
 
 coverage-report: ## Combine and report coverage
 	uv run $(PYTHON_ARG) coverage combine