fix: resolve AttributeError in Encoding.is_special_token()

[HrushiYadav]

Summary

Fixes #536

• is_special_token() references self._special_token_values which is never defined, causing AttributeError on every call
• Changed to self._special_tokens.values() which correctly checks if the token id exists among the special token values
• Added test for is_special_token with both special and regular tokens

Verification

import tiktoken
enc = tiktoken.get_encoding("cl100k_base")

# Before fix: AttributeError
# After fix:
eot = enc.encode_single_token("<|endoftext|>")
enc.is_special_token(eot)  # True

hello = enc.encode("hello")[0]
enc.is_special_token(hello)  # False

Test plan

• Verified bug exists on installed tiktoken (AttributeError)
• Verified fix returns correct results (True for special, False for regular)
• Added test in test_encoding.py

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