fix: improve consistent error handling and exit codes in wallet balance script

[JHON12091986]

Summary

This PR improves the error handling consistency in the wallet balance check script (tools/rustchain_wallet_cli.py) by adding standardized exit codes and specific error handling for all failure paths.

Changes

• Add EXIT_SUCCESS, EXIT_USAGE_ERROR, EXIT_NETWORK_ERROR, EXIT_BAD_RESPONSE, EXIT_WALLET_NOT_FOUND, EXIT_AUTH_ERROR, EXIT_UNKNOWN_ERROR codes
• Improve cmd_balance() with specific error handling for:
  • Network errors (timeout, connection refused)
  • HTTP 404 (wallet not found)
  • Non-200 responses
  • Malformed JSON responses
  • Missing "amount_rtc" field
• Add proper error messages to stderr for all failure paths
• No behavior change on happy path

Testing

• Verified happy path returns balance with exit code 0
• Verified network error returns exit code 2 with stderr message
• Verified 404 returns exit code 4 with stderr message

Closes

Closes Rustchain#7889
Closes #16253

Bounty

RTC wallet: RTC921ffc4353965fe09d946c1d826e398f0437d97e
Miner ID: JHON12091986

[github-actions]

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[github-actions]

RTC Reward

This merged PR earned 5 RTC — sent to RTC921ffc4353965fe09d946c1d826e398f0437d97e.

RustChain Bounty Program

[Scottcjn]

Merged — clean work: named exit-code constants (EXIT_USAGE_ERROR, EXIT_NETWORK_ERROR, etc.) replacing the magic return 2s, exactly what #16253 asked for. Awarded 7 RTC. Post your RTC wallet (RTC...) here and I'll send it. — Sophia, Elyan Labs

[Scottcjn]

Hold on the payout — the wallet you posted (RTC921ffc…d97e) already belongs to a different contributor (jjb9707, paid earlier for other work). I can't send your 7 RTC to someone else's address. Please post your own RTC wallet — generate one with the RustChain wallet tool if you don't have it yet — and I'll send it there. Just want your award to reach you. — Sophia