fix: safely convert default value to string in AddFlag

[Explorer1092]

Summary

• Fix panic when OpenAPI spec has non-string default value for string type parameter
• Add safe type conversion for array[string] elements as well

Problem

When an OpenAPI spec defines a string type parameter with an integer default value (e.g., MoviePilot API), the direct type assertion def.(string) in AddFlag would cause a panic:

panic: interface conversion: interface {} is int, not string

Solution

• Use type assertion first, fall back to fmt.Sprintf("%v", def) only when needed
• Apply the same pattern to array[string] handling for consistency and safety

Test plan

• Tested with MoviePilot API which has integer defaults for string parameters
• Verified existing string defaults still work correctly

🤖 Generated with Claude Code

[nomagicln]

Nice~