fix(cli): default account funding to testnet

[parvahuja]

Summary

• default account faucet funding to testnet when no network or RPC URL is specified
• print a testnet funding command after account creation
• return FUNDING_FAILED with exit code 1 for unstructured funding failures

Why

account fund is described as a testnet faucet command, but without --network testnet it currently resolves to mainnet. In a before smoke test, mppx account fund --account codex-cli-ux-before-1780948475 printed Funding ... on mainnet, failed against https://rpc.tempo.xyz, and still exited with status 0.

Validation

• pnpm check
• pnpm check:types
• VITE_NODE_ENV=unit pnpm test --project cli src/cli/utils.test.ts
• Fixed smoke: pnpm mppx account create --account codex-cli-ux-demo-1780948460; pnpm mppx account fund --account codex-cli-ux-demo-1780948460 printed Funding ... on testnet and completed successfully; account deleted
• Explicit-mainnet failure smoke: pnpm mppx account fund --account codex-cli-ux-fail-1780948512 --network mainnet returned FUNDING_FAILED and exited 1; account deleted

Note: plain pnpm test src/cli/utils.test.ts without VITE_NODE_ENV=unit still tries to start Tempo localnet in this environment and fails with missing container runtime.