refactor: simplify command execution and remove PTY handling (#1641)

# What does this PR do?

A PTY is unnecessary for interactive mode since `subprocess.run()`
already inherits the calling terminal’s stdin, stdout, and stderr,
allowing natural interaction. Using a PTY can introduce unwanted side
effects like buffering issues and inconsistent signal handling. Standard
input/output is sufficient for most interactive programs.

This commit simplifies the command execution by:

1. Removing PTY-based execution in favor of direct subprocess handling
2. Consolidating command execution into a single run_command function
3. Improving error handling with specific subprocess error types
4. Adding proper type hints and documentation
5. Maintaining Ctrl+C handling for graceful interruption

## Test Plan

```
llama stack run
```

Signed-off-by: Sébastien Han <seb@redhat.com>

.github/workflows/integration-tests.yml

@@ -56,8 +56,7 @@ jobs:
           INFERENCE_MODEL: "meta-llama/Llama-3.2-3B-Instruct"
         run: |
           source .venv/bin/activate
-          # TODO: use "llama stack run"
-          nohup uv run python -m llama_stack.distribution.server.server --yaml-config ./llama_stack/templates/ollama/run.yaml > server.log 2>&1 &
+          nohup uv run llama stack run ./llama_stack/templates/ollama/run.yaml --image-type venv > server.log 2>&1 &
 
       - name: Wait for Llama Stack server to be ready
         run: |