fix: handle non-JSON HTTP error responses in wallet CLI commands

[wsimon1982]

Fixes #765

Problem

cmd_balance, cmd_history, cmd_miners, cmd_epoch, and cmd_send in tools/rustchain_wallet_cli.py all called r.json() directly without checking whether the HTTP response body contained valid JSON.

When the node returns a non-JSON error (e.g. 502 Bad Gateway with an HTML body, plain-text 503, etc.) a requests.exceptions.JSONDecodeError propagated to the user with an opaque message:

Error: Expecting value: line 1 column 1 (char 0)

Solution

Introduce a _safe_json(r) helper that catches JSONDecodeError and prints a descriptive HTTP status message:

Error: Server returned HTTP 502 with non-JSON body (check node URL / connectivity)

Returns (None, 1) so callers propagate the error exit-code cleanly. All five affected commands updated to use _safe_json.

Testing

export RUSTCHAIN_NODE_URL=https://httpbin.org/status/502
python -m tools.rustchain_wallet_cli balance RTCabc123
# Before: Error: Expecting value: line 1 column 1 (char 0)
# After:  Error: Server returned HTTP 502 with non-JSON body (check node URL / connectivity)

[github-actions]

Welcome to RustChain! Thanks for your first pull request.

Before we review, please make sure:

• Your PR has a BCOS-L1 or BCOS-L2 label
• New code files include an SPDX license header
• You've tested your changes against the live node

Bounty tiers: Micro (1-10 RTC) | Standard (20-50) | Major (75-100) | Critical (100-150)

A maintainer will review your PR soon. Thanks for contributing!

[Scottcjn]

Solid improvement. _safe_json() helper properly handles HTML error pages (502/503 from nginx) instead of crashing with JSONDecodeError. Clean, consistent application across all 5 wallet commands. Type annotation uses from __future__ import annotations already present in the file.

Verdict: MERGE — 15 RTC bounty (standard improvement).

🤖 Reviewed with GPT-5.4 + Claude Opus dual-brain analysis