feat(supported): return CAIP-2 chain IDs for v2 protocol

[kbak]

Summary

• The /supported endpoint only returned v1-format entries with network names (e.g. "base", "solana") and x402Version: 1
• V2 clients expect CAIP-2 chain IDs (e.g. "eip155:8453", "solana:mainnet") with x402Version: 2
• Each provider (EVM, Solana, Aptos) now returns both a v1 entry and a v2 entry, making the response backward compatible

Example response (before)

{"kinds": [{"x402Version": 1, "scheme": "exact", "network": "base"}]}

Example response (after)

{"kinds": [
  {"x402Version": 1, "scheme": "exact", "network": "base"},
  {"x402Version": 2, "scheme": "exact", "network": "eip155:8453"}
]}

Files changed

• src/chain/evm.rs — add v2 entry in supported()
• src/chain/solana.rs — add v2 entry in supported()
• src/chain/aptos.rs — add v2 entry in supported()
• src/types.rs — add X402Version::V2 variant with serde support
• src/network.rs — add CAIP-2 format test

Test plan

• cargo test --lib — 143 tests pass
• cargo clippy — no new warnings
• New tests verify v1+v2 entries for Aptos mainnet and testnet
• New test verifies JSON wire format (x402Version serializes as 2)
• New test verifies all Network variants produce valid CAIP-2 strings

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