fix(scripts): exit non-zero when Friendbot funding fails

[Yuren0312]

Problem

scripts/02-fund-accounts.js (#31):

• fund() called res.json() unconditionally. Friendbot answers some failures with HTML or plain text, so the step crashed with a SyntaxError instead of reporting the failure.
• The script always exited 0, even when no account was funded, so a failed step 2 looked successful to anything chained after it.

Change

• Read the body as text first and parse JSON from it (readBody), so a non-JSON error body is reported instead of crashing the step.
• Validate every key with StrKey.isValidEd25519PublicKey before contacting Friendbot; invalid keys abort with exit 1 and Friendbot is never called.
• Treat an already funded response as success — the account already holds testnet XLM.
• Count failures and exit 1 when any account could not be funded; the "Next →" footer only prints on full success.
• test/fund-accounts.test.cjs covers the exit-code contract end-to-end (the script is spawned with fetch stubbed by test/friendbot-fetch-stub.mjs): success, already-funded, non-JSON error body, network failure, partial failure, invalid keys and missing arguments.
• README: document the new exit status for step 2.

Verification

• The 7 new cases in test/fund-accounts.test.cjs pass (node --test, same command as npm test); they assert exit codes and captured output, and that Friendbot is never contacted for invalid keys.
• node --check passes for every script.

Fixes #31