Fix array parameter serialization for tollfree verification requests

[Copilot]

The tollfree verification API was returning "Missing required parameter" errors for array parameters (UseCaseCategories and OptInImageUrls) even when they were properly included in the request. This issue occurred specifically when using sub-accounts.

The root cause was that array parameters were being serialized as repeated form parameters (param=A&param=B&param=C) instead of comma-separated strings (param=A,B,C) which is the format expected by the Twilio API.

Changes Made

• Modified TollfreeVerificationList#create to serialize array parameters as comma-separated strings
• Modified TollfreeVerificationContext#update to use the same serialization approach
• Added comprehensive test coverage for array parameter handling

Before (causing API errors):

use_case_categories: ['ACCOUNT_NOTIFICATIONS', 'EVENTS']
# Serialized as: UseCaseCategories=ACCOUNT_NOTIFICATIONS&UseCaseCategories=EVENTS

After (working correctly):

use_case_categories: ['ACCOUNT_NOTIFICATIONS', 'EVENTS'] 
# Serialized as: UseCaseCategories=ACCOUNT_NOTIFICATIONS,EVENTS

The fix handles all edge cases correctly:

• Arrays are joined with commas
• Non-array values pass through unchanged
• Empty arrays become empty strings
• nil and :unset values are preserved

Fixes #674.

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