fix(tests): add --cov-append to gapic-generator and proto-plus to preserve monorepo coverage (#17603)

The cover presubmit job failed in
https://github.com/googleapis/google-cloud-python/actions/runs/28464524723/job/84362399849?pr=17600
with `No data to report` for `db-dtypes`.

While most packages correctly use the `--cov-append` flag, both
`gapic-generator` and `proto-plus` were missing it in their respective
`noxfile.py` configurations. When their test suites executed,
`pytest-cov` fell back to its default behavior and truncated the shared
coverage file. This silently wiped out the coverage data of any packages
that ran before them (such as `db-dtypes`), causing the downstream
coverage aggregation job to fail.

Changes

- Added the `--cov-append` flag to the pytest commands in
`packages/gapic-generator/noxfile.py`.
- Added the `--cov-append` flag to the pytest commands in
`packages/proto-plus/noxfile.py`.
- Use `"--cov-fail-under=0",` when running `pytest` in
`gapic-generator`. Each package should specify `fail_under` in
`.coveragerc`

This ensures these packages safely add their coverage data to the shared
file without destroying the results of any packages that were tested
before them.

Author: parthea

packages/gapic-generator/noxfile.py

@@ -82,9 +82,10 @@ def unit(session):
                 "-vv",
                 "-n=auto",
                 "--cov=gapic",
+                "--cov-append",
                 "--cov-config=.coveragerc",
                 "--cov-report=term",
-                "--cov-fail-under=100",
+                "--cov-fail-under=0",
                 path.join("tests", "unit"),
             ]
         ),

packages/proto-plus/noxfile.py

@@ -82,6 +82,7 @@ def unit(session, implementation):
             session.posargs  # Coverage info when running individual tests is annoying.
             or [
                 "--cov=proto",
+                "--cov-append",
                 "--cov-config=.coveragerc",
                 "--cov-report=term",
                 "--cov-report=html",