fix: handle missing prompts in custom commands

[abeatrix]

context: https://sourcegraph.slack.com/archives/C04MSD3DP5L/p1706656724966049

fix: handle commands missing prompts when building map

Issue: Currently, an invalid entry in the cody.json will cause custom commands not showing up in the command menu.

Fix: Updated the get-commands utility to skip adding commands to the map if they don't contain a prompt property. It updates the command type and adds null checking before accessing the prompt property.

This prevents crashes when building the custom command map if a command is missing the expected prompt field.

Test plan

Updated both the unit test and e2e test to ensure an invalid custom command entry would not break the custom command at build time.

Create a custom command entry without adding a prompt in your cody.json.

Before: all your custom command from that file will not show up in the custom command menu.

After: only the invalid ones will not show up in the menu, while the valid ones do.