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>

llama_stack/distribution/build.py

@@ -6,7 +6,6 @@
 
 import importlib.resources
 import logging
-import sys
 from pathlib import Path
 from typing import Dict, List
 
@@ -15,7 +14,7 @@ from termcolor import cprint
 
 from llama_stack.distribution.datatypes import BuildConfig, Provider
 from llama_stack.distribution.distribution import get_provider_registry
-from llama_stack.distribution.utils.exec import run_command, run_with_pty
+from llama_stack.distribution.utils.exec import run_command
 from llama_stack.distribution.utils.image_types import LlamaStackImageType
 from llama_stack.providers.datatypes import Api
 
@@ -123,11 +122,7 @@ def build_image(
     if special_deps:
         args.append("#".join(special_deps))
 
-    is_terminal = sys.stdin.isatty()
-    if is_terminal:
-        return_code = run_with_pty(args)
-    else:
-        return_code = run_command(args)
+    return_code = run_command(args)
 
     if return_code != 0:
         log.error(