Fixes three bugs that caused `coder update` to always re-prompt for
multi-select (`list(string)`) parameters instead of reusing previous
build values:
1. **`isValidTemplateParameterOption` failed for multi-select values**
(`cli/parameterresolver.go`): It compared the entire JSON array string
(e.g. `["vim","emacs"]`) against individual option values, which never
matched. Now parses the JSON array and validates each element
separately.
2. **`RichParameter` ignored previous build value for multi-select**
(`cli/cliui/parameter.go`): The `list(string)` branch always used the
template's default value instead of the `defaultValue` argument (which
carries the previous build's value). Now uses `defaultValue` when
available, falling back to the template default.
3. **Pre-existing crash when `list(string)` has no default value**
(`cli/cliui/parameter.go`): `json.Unmarshal` on an empty string caused
`unexpected end of JSON input`. Now skips unmarshaling when the default
source is empty.
Fixes#19956
Kacper Sawicki