Fix acronym handling in camelCase→snake_case conversion

[Copilot]

The normalize_field_name function used naive character-by-character conversion that mangled acronyms, producing false-positive drift reports (APIResponse → a_p_i_response instead of api_response).

Changes

• Replaced single-pass character iteration with two-pass regex approach:
  1. Insert underscore before uppercase followed by lowercase: APIResponse → API_Response
  2. Insert underscore between lowercase/digit and uppercase: getUserID → get_User_ID
  3. Lowercase the result

Examples

# Before (naive)
normalize_field_name("APIResponse")      # "a_p_i_response" ❌
normalize_field_name("HTTPSConnection")  # "h_t_t_p_s_connection" ❌

# After (regex-based)
normalize_field_name("APIResponse")      # "api_response" ✓
normalize_field_name("HTTPSConnection")  # "https_connection" ✓
normalize_field_name("getUserID")        # "get_user_id" ✓

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