util: Prevent panic when parsing a malformed shell variable

[39ali]

Objective

to_cmd_variable and to_powershell_variable removed the last byte of a
${...} argument assuming it was the closing brace, without checking one was
there

// If the input starts with "${", remove the trailing "}"
format!("$env:{}", &var_str[..var_str.len() - 1])

Solution

Use strip_suffix('}') and pass the input through when it isn't a variable reference

Testing

added tests to cover this

To reproduce on Windows, add a context server with a malformed argument to settings.json:

"context_servers": {
  "crash-repro": {
    "command": "does-not-matter",
    "args": ["${"]
  }
}
} 

Opening the agent panel and zed will crash

Release Notes:

• Fixed a panic when converting a malformed ${ shell variable reference on Windows

[SomeoneToIgnore]

Thanks, one nit to fix and we're ready to merge.

[SomeoneToIgnore]

This comment belongs to input.into branch, let's keep them together.