test: Add one-step integration testing with server auto-start (#2580)

## Summary

Add support for `server:<config>` format in `--stack-config` option to
enable seamless one-step integration testing. This eliminates the need
to manually start servers in separate terminals before running tests.

## Key Features

- **Auto-start server**: Automatically launches `llama stack run
<config>` if target port is available
- **Smart reuse**: Reuses existing server if port is already occupied  
- **Health check polling**: Waits up to 2 minutes for server readiness
via `/v1/health` endpoint
- **Custom port support**: Use `server:<config>:<port>` for non-default
ports
- **Clean output**: Server runs quietly in background without cluttering
test output
- **Backward compatibility**: All existing `--stack-config` formats
continue to work

## Usage Examples

```bash
# Auto-start server with default port 8321
pytest tests/integration/inference/ --stack-config=server:fireworks

# Use custom port
pytest tests/integration/safety/ --stack-config=server:together:8322

# Run multiple test suites seamlessly  
pytest tests/integration/inference/ tests/integration/agents/ --stack-config=server:starter
```

## Implementation Details

- Enhanced `llama_stack_client` fixture with server management
- Updated documentation with cleaner organization and comprehensive
examples
- Added utility functions for port checking, server startup, and health
verification

## Test Plan

- Verified server auto-start when port 8321 is available
- Verified server reuse when port 8321 is occupied
- Tested health check polling via `/v1/health` endpoint
- Confirmed custom port configuration works correctly
- Verified backward compatibility with existing config formats

## Before/After Comparison

**Before (2 steps):**
```bash
# Terminal 1: Start server manually
llama stack run fireworks --port 8321

# Terminal 2: Wait for startup, then run tests  
pytest tests/integration/inference/ --stack-config=http://localhost:8321
```

**After (1 step):**
```bash
# Single command handles everything
pytest tests/integration/inference/ --stack-config=server:fireworks  
```

tests/integration/README.md

@@ -9,7 +9,9 @@ pytest --help
 ```
 
 Here are the most important options:
-- `--stack-config`: specify the stack config to use. You have three ways to point to a stack:
+- `--stack-config`: specify the stack config to use. You have four ways to point to a stack:
+  - **`server:<config>`** - automatically start a server with the given config (e.g., `server:fireworks`). This provides one-step testing by auto-starting the server if the port is available, or reusing an existing server if already running.
+  - **`server:<config>:<port>`** - same as above but with a custom port (e.g., `server:together:8322`)
   - a URL which points to a Llama Stack distribution server
   - a template (e.g., `fireworks`, `together`) or a path to a `run.yaml` file
   - a comma-separated list of api=provider pairs, e.g. `inference=fireworks,safety=llama-guard,agents=meta-reference`. This is most useful for testing a single API surface.
@@ -26,12 +28,39 @@ Model parameters can be influenced by the following options:
 Each of these are comma-separated lists and can be used to generate multiple parameter combinations. Note that tests will be skipped
 if no model is specified.
 
-Experimental, under development, options:
-- `--record-responses`: record new API responses instead of using cached ones
-
-
 ## Examples
 
+### Testing against a Server
+
+Run all text inference tests by auto-starting a server with the `fireworks` config:
+
+```bash
+pytest -s -v tests/integration/inference/test_text_inference.py \
+   --stack-config=server:fireworks \
+   --text-model=meta-llama/Llama-3.1-8B-Instruct
+```
+
+Run tests with auto-server startup on a custom port:
+
+```bash
+pytest -s -v tests/integration/inference/ \
+   --stack-config=server:together:8322 \
+   --text-model=meta-llama/Llama-3.1-8B-Instruct
+```
+
+Run multiple test suites with auto-server (eliminates manual server management):
+
+```bash
+# Auto-start server and run all integration tests
+export FIREWORKS_API_KEY=<your_key>
+
+pytest -s -v tests/integration/inference/ tests/integration/safety/ tests/integration/agents/ \
+   --stack-config=server:fireworks \
+   --text-model=meta-llama/Llama-3.1-8B-Instruct
+```
+
+### Testing with Library Client
+
 Run all text inference tests with the `together` distribution:
 
 ```bash