[P0][M] Switch zkx402 to CAIP-2 network identifiers

[markballew]

Summary

Standardize on CAIP-2 network identifiers across config, metadata, and tests.

Acceptance criteria

• Replace legacy network strings (e.g. base, base-sepolia, solana-devnet) with CAIP-2 forms:
  • eip155:8453, eip155:84532, solana:<genesisHash>
• Demo + package accept and emit CAIP-2 networks
• No runtime paths depend on legacy names

Notes

• x402 v2 uses CAIP-2 identifiers; v1 compatibility can remain briefly but v2 should be the default.