test: Measure and track code coverage (#2636)

# What does this PR do?
<!-- Provide a short summary of what this PR does and why. Link to
relevant issues if applicable. -->
- Added coverage badge to README. - [See my
fork](https://github.com/ChristianZaccaria/llama-stack)
- Added a GitHub Actions workflow that runs the tests and updates the
coverage badge. - [See
run](4574811323)
- Documented steps in `testing.md` for running the tests locally, and
viewing the `html` report.
- Excluded non-essential files from coverage reporting to provide a more
accurate measurement.

Automatically created PR to update coverage badge:
https://github.com/ChristianZaccaria/llama-stack/pull/9

# Note for reviewers
1. Currently the coverage report shows a 45% coverage. Wondering if
there are other files or directories that should also be excluded from
the report to increase the percentage. The directories with the least
test coverage are `llama_stack/cli`, `llama_stack/models`, and
`llama_stack/ui`. - Should we exclude these?
2. **[Required]** The `GITHUB_TOKEN` should have write permissions to
open a PR to update the coverage badge.

# GitHub Issue
<!-- If resolving an issue, uncomment and update the line below -->
<!-- Closes #[issue-number] -->
Closes #2355 

## Test Plan
<!-- Describe the tests you ran to verify your changes with result
summaries. *Provide clear instructions so the plan can be easily
re-executed.* -->
The `testing.md` file describes how to run the unit tests locally.

.github/workflows/coverage-badge.yml

@@ -0,0 +1,57 @@
+name: Coverage Badge
+
+on:
+  push:
+    branches: [ main ]
+    paths:
+      - 'llama_stack/**'
+      - 'tests/unit/**'
+      - 'uv.lock'
+      - 'pyproject.toml'
+      - 'requirements.txt'
+      - '.github/workflows/unit-tests.yml'
+      - '.github/workflows/coverage-badge.yml' # This workflow
+  workflow_dispatch:
+
+jobs:
+  unit-tests:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+
+      - name: Install dependencies
+        uses: ./.github/actions/setup-runner
+
+      - name: Run unit tests
+        run: |
+          ./scripts/unit-tests.sh
+
+      - name: Coverage Badge
+        uses: tj-actions/coverage-badge-py@1788babcb24544eb5bbb6e0d374df5d1e54e670f # v2.0.4
+
+      - name: Verify Changed files
+        uses: tj-actions/verify-changed-files@a1c6acee9df209257a246f2cc6ae8cb6581c1edf # v20.0.4
+        id: verify-changed-files
+        with:
+          files: coverage.svg
+
+      - name: Commit files
+        if: steps.verify-changed-files.outputs.files_changed == 'true'
+        run: |
+          git config --local user.email "github-actions[bot]@users.noreply.github.com"
+          git config --local user.name "github-actions[bot]"
+          git add coverage.svg
+          git commit -m "Updated coverage.svg"
+
+      - name: Create Pull Request
+        if: steps.verify-changed-files.outputs.files_changed == 'true'
+        uses: peter-evans/create-pull-request@271a8d0340265f705b14b6d32b9829c1cb33d45e # v7.0.8
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          title: "ci: [Automatic] Coverage Badge Update"
+          body: |
+            This PR updates the coverage badge based on the latest coverage report.
+
+            Automatically generated by the [workflow coverage-badge.yaml](.github/workflows/coverage-badge.yaml)
+          delete-branch: true

.github/workflows/unit-tests.yml

@@ -36,7 +36,7 @@ jobs:
 
       - name: Run unit tests
         run: |
-          PYTHON_VERSION=${{ matrix.python }} ./scripts/unit-tests.sh --cov=llama_stack --junitxml=pytest-report-${{ matrix.python }}.xml --cov-report=html:htmlcov-${{ matrix.python }}
+          PYTHON_VERSION=${{ matrix.python }} ./scripts/unit-tests.sh --junitxml=pytest-report-${{ matrix.python }}.xml
 
       - name: Upload test results
         if: always()