fix: search_token returns None for chainless queries due to incorrect guard placement

[bigeez]

Bug

search_token in onchain/tokens/metadata.py incorrectly returns None
for any token search that doesn't pass a chain argument.

The guard if chain not in SUPPORTED_CHAINS: return None was at the same
indentation level as if chain:, meaning it always ran even when chain was
None — killing chainless fallback searches silently.

Fix

Indented the SUPPORTED_CHAINS check inside the if chain: block so it
only runs when a chain is actually provided. Chainless searches now correctly
fall through to _search_token_on_dexscreener.

Testing

• Token search with a valid chain still filters correctly
• Token search with no chain now reaches the DexScreener fallback as intended