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.

tests/unit/README.md

@@ -1,9 +1,17 @@
 # Llama Stack Unit Tests
 
+## Unit Tests
+
+Unit tests verify individual components and functions in isolation. They are fast, reliable, and don't require external services.
+
+### Prerequisites
+
+1. **Python Environment**: Ensure you have Python 3.12+ installed
+2. **uv Package Manager**: Install `uv` if not already installed
+
 You can run the unit tests by running:
 
 ```bash
-source .venv/bin/activate
 ./scripts/unit-tests.sh [PYTEST_ARGS]
 ```
 
@@ -19,3 +27,21 @@ If you'd like to run for a non-default version of Python (currently 3.12), pass
 source .venv/bin/activate
 PYTHON_VERSION=3.13 ./scripts/unit-tests.sh
 ```
+
+### Test Configuration
+
+- **Test Discovery**: Tests are automatically discovered in the `tests/unit/` directory
+- **Async Support**: Tests use `--asyncio-mode=auto` for automatic async test handling
+- **Coverage**: Tests generate coverage reports in `htmlcov/` directory
+- **Python Version**: Defaults to Python 3.12, but can be overridden with `PYTHON_VERSION` environment variable
+
+### Coverage Reports
+
+After running tests, you can view coverage reports:
+
+```bash
+# Open HTML coverage report in browser
+open htmlcov/index.html  # macOS
+xdg-open htmlcov/index.html  # Linux
+start htmlcov/index.html  # Windows
+```