fix(form): fix infinite recursion when decoding self-referenced structs

[tatang26]

Summary

Fixes infinite recursion bug when decoding self-referenced structs in form data.

Problem

The form decoder recursively processed pointer fields even when no form data existed for them, causing infinite recursion on self-referencing types like:

type Node struct {
    ID     string
    Parent *Node  // Would recurse infinitely
}

Solution

Added form data existence check before recursing into struct pointer fields (form/decode.go:73-92):

• Only processes nested structs when matching form data exists
• Checks for form keys with the field's prefix
• Properly handles anonymous fields

Testing

• ✅ Self-referencing structs with/without data
• ✅ Multiple nesting levels (3+ deep)
• ✅ Tree structures and bidirectional links
• ✅ Added benchmark for performance validation