fix(cli): reuse multi-select parameter values on workspace update (#22261)

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

cli/cliui/parameter.go

@@ -30,9 +30,15 @@ func RichParameter(inv *serpent.Invocation, templateVersionParameter codersdk.Te
 		_, _ = fmt.Fprint(inv.Stdout, "\033[1A")
 
 		var defaults []string
-		err = json.Unmarshal([]byte(templateVersionParameter.DefaultValue), &defaults)
-		if err != nil {
-			return "", err
+		defaultSource := defaultValue
+		if defaultSource == "" {
+			defaultSource = templateVersionParameter.DefaultValue
+		}
+		if defaultSource != "" {
+			err = json.Unmarshal([]byte(defaultSource), &defaults)
+			if err != nil {
+				return "", err
+			}
 		}
 
 		values, err := RichMultiSelect(inv, RichMultiSelectOptions{