feat(go): add BSC support and explicit network selection

[deszhou]

Description

• add BSC mainnet/testnet metadata (USDC address, decimals, token name) to the Go middleware
• require callers to specify networks via WithNetwork, defaulting to base-sepolia when omitted
• update proxy demo, examples, and config to use the new network flow and run amounts through high-precision scaling

Tests

Checklist

• [x] I have formatted and linted my code
• [x] All new and existing tests pass
• [x] My commits are signed (required for merge) -- you may need to rebase if you initially pushed unsigned commits

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1

[vercel[bot]]

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