feat(php): add support for deep object query parameters

[iamnamananand996]

Description

Refs: Requested by @iamnamananand996

Adds support for deep object query parameters in the PHP SDK generator. Nested objects in query parameters are now encoded using bracket notation (e.g., filter[user][name]=john&filter[user][age]=30), matching the OpenAPI 3.0 deepObject serialization style.

This brings PHP in line with Python, Java, and C# generators which already support this feature.

Link to Devin run: https://app.devin.ai/sessions/64b3914c128f4cfa957eaab189b9c426

Changes Made

• Added flattenQueryParams() method to RawClient.Template.php that recursively flattens nested arrays into query parameter format with bracket notation
• Modified encodeQuery() to use the new flattening method
• Added 3 unit tests covering:
  • Simple deep object query parameters (filter[name]=john)
  • Deeply nested objects (filter[user][name]=john)
  • Arrays of objects (objects[key]=hello&objects[value]=world)
• Updated versions.yml with version 1.26.0 changelog entry
• Updated README.md generator (if applicable) - N/A

Testing

• Unit tests added/updated
• Manual testing completed (verified with mock server that encoding produces correct OpenAPI deepObject format)

Updates Since Last Revision

• Merged main branch to resolve merge conflict in versions.yml (incorporated v1.25.4 fix for wire tests)

Human Review Checklist

• Verify flattenQueryParams correctly distinguishes between associative arrays (objects) and sequential arrays (lists) using isSequential()
• Confirm empty array handling is correct (empty arrays treated as sequential, resulting in no query params)
• Verify test assertions match expected OpenAPI deepObject serialization format (brackets URL-encoded as %5B and %5D)

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