feat(config-gen): emit Soroban RPC env vars for soroban-defi template

[Henrichy]

• Add DEFAULT_SOROBAN_RPC_URL constant for mainnet/testnet resolution
• Implement resolveSorobanRpcUrl helper for consistent fallback logic
• Update buildStellarEnvVars to accept family parameter and emit
  NEXT_PUBLIC_SOROBAN_RPC_URL and NEXT_PUBLIC_CONTRACT_ADDRESSES
• Update env-template-generator.ts to include required keys in .env.example
• Update generateStellarConfigFile to use requiresSoroban/usesAssetPairs/usesContractAddresses guards
• Fix test failure and ensure all tests pass
• Closes Extend stellar-config-generator to emit Soroban RPC env vars for soroban-defi template #479

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