fix: scope Codecov report to product code, excluding test files

CI runs `pytest --cov=sagemaker` per sub-package and uploads to Codecov,
but coverage is not scoped to `src/` product code. Executed test-support
scripts (notably `*/tests/data/*.py` training entry-scripts and model
definitions) get recorded and uploaded.

With no `codecov.yml` ignore list, these dominate the report: recent
commits on master show ~98% of the measured files/lines are test files
rather than product code (e.g. commit 3f810c6: 285 of 290 reported
files are under tests/). This inflates the reported project coverage
(~88-90%) far above the true product-code coverage and makes the number
unreliable and unstable across commits.

Add a repo-root `codecov.yml` that scopes the report to product code via
an `ignore:` list (tests, conftest, test-data scripts, codegen and
workflow_helper utilities), and defines project/patch status targets and
`after_n_builds: 4` so the percentage is computed over all four
sub-package uploads. Validated with the Codecov config validator.

This follows how peer projects configure Codecov (scikit-learn ignores
conftest/externals; aws-cdk ignores non-product paths).

Author: jam-jee

codecov.yml

@@ -0,0 +1,51 @@
+# Codecov configuration for the sagemaker-python-sdk (v3) monorepo.
+# https://docs.codecov.com/docs/codecovyml-reference
+#
+# Why this file exists:
+#   CI runs `pytest --cov=sagemaker` per sub-package and uploads to Codecov. Because
+#   coverage is not scoped to the `src/` product code, executed test-support scripts
+#   (notably `*/tests/data/*.py` training entry-scripts and model definitions) are
+#   recorded and uploaded. With no ignore list these dominated the report — recent
+#   commits show ~98% of the measured files/lines were test files, not product code,
+#   which inflates the reported coverage and makes the number meaningless.
+#
+# The `ignore:` list below scopes the Codecov report to product code only, matching
+# how peer projects configure Codecov (e.g. scikit-learn ignores conftest/externals;
+# aws-cdk ignores non-product paths).
+
+codecov:
+  # Wait for all four sub-package unit-test jobs to upload before finalizing, so the
+  # project % is computed over the complete report rather than whichever job lands first.
+  notify:
+    after_n_builds: 4
+    wait_for_ci: true
+
+coverage:
+  status:
+    project:
+      default:
+        # Do not let overall product coverage regress.
+        target: auto
+        threshold: 1%
+    patch:
+      default:
+        # New/changed product lines should be reasonably covered.
+        target: 80%
+        threshold: 1%
+
+comment:
+  layout: "header, diff, flags, components"
+  require_changes: true
+
+# Paths excluded from coverage measurement/reporting. These are test code, test
+# fixtures/data scripts, and build/codegen helpers — not shippable product code.
+ignore:
+  - "**/tests/**"
+  - "**/test_*.py"
+  - "**/conftest.py"
+  - "sagemaker-*/test_script.py"
+  - "**/workflow_helper/**"
+  - "**/tools/*_codegen.py"
+  - "**/setup.py"
+  - "docs/**"
+  - "v3-examples/**"