Fix double encoding in nested query parameters at 3+ levels

[Copilot]

The stringify function was double-encoding nested query parameters beyond 2 levels deep. Encoded keys were passed as prefixes to recursive calls, causing re-encoding at each level.

Example of the bug:

const url = new DynamicURL("https://example.com");
url.setQueryParams({ 
  level1: { level2: { level3: "value" } }
});

// Before: level1%255Blevel2%255D%5Blevel3%5D=value
//         (decodes to: level1%5Blevel2%5D[level3])
// After:  level1%5Blevel2%5D%5Blevel3%5D=value
//         (decodes to: level1[level2][level3])

Changes:

• Build unencoded key path during recursion
• Encode once at leaf level only
• Add regression test for 3+ level nesting

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