Fix: Improve error handling for missing SSH client and add unit tests for `shell` command

[asideofcode-dev]

Summary

NOTE: The first 3 commits are based off the preceding PR https://github.com/brevdev/brev-cli/pull/198.

Resolves #185

This PR addresses the issue where running the brev shell command in a Docker environment without an installed SSH client would cause a panic. Additionally, it improves the error handling when the specified workspace cannot be found.

Key changes

1. SSH Client Check:

   • Added a check to ensure that the SSH client is installed and available in the system's PATH before attempting to establish a connection. If the SSH client is not found, a clear and actionable error message is returned.

2. Unit Tests:

   • Added unit tests to verify the behavior when the SSH client is not installed, ensuring that the error is properly caught and reported.
   • Added a test case to validate that an appropriate error is raised when the workspace is not found.

3. Code Cleanup:

   • Performed minor formatting improvements and removed unnecessary whitespace for cleaner code.

Impact

• Users running brev shell in environments without an SSH client will now receive a clear error message guiding them to install the required software.
• Ensures robustness in handling edge cases where the specified workspace cannot be found.