fix(mint): catch backend errors during startup status check

[KvngMikey]

Closes #518.

Summary

Ledger._check_backends() already renders an unhealthy backend into a readable message and exits cleanly — but only when the backend returns a StatusResponse carrying error_message. Backends that raise instead bypass that path entirely: the exception propagates through startup_ledger() to start_mint() to the FastAPI lifespan, and the operator gets the raw stack trace the issue describes.

Reproduced on current main by pointing MINT_BACKEND_BOLT11_SAT=CLNRestWallet at an unreachable host:

INFO | Using CLNRestWallet backend for method: 'bolt11' and unit: 'sat'
httpx.ConnectError: All connection attempts failed

Changes

Rather than patch each backend, this normalises a raised exception into the StatusResponse contract at the single point where startup consumes it.

Reporting failure two different ways (return vs. raise) is what let the backends drift apart in the first place. Handling both where they are read means a backend added later is covered without its author having to know this rule exists, which is what makes this a fix rather than four patches.

The exception type is kept in the operator-facing message so the cause stays identifiable, and the full traceback is logged at DEBUG for maintainers diagnosing an unexpected failure rather than an unreachable host.

Result

ERROR | The backend for CLNRestWallet isn't working properly:
'ConnectError: All connection attempts failed'

[Copilot]

Pull request overview

This PR improves mint startup robustness by ensuring backend status-check failures raised as exceptions are converted into the existing StatusResponse error contract, preventing noisy stack traces from leaking to operators during FastAPI lifespan startup.

Changes:

• Wraps backend .status() calls during Ledger._check_backends() in a try/except to normalize raised exceptions into a StatusResponse(error_message=...).
• Logs the full traceback at DEBUG while keeping the operator-facing failure message concise and consistent.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.66%. Comparing base (6eed7b2) to head (6cb7f2b).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1124      +/-   ##
==========================================
+ Coverage   74.59%   74.66%   +0.07%     
==========================================
  Files         113      113              
  Lines       12760    12765       +5     
==========================================
+ Hits         9518     9531      +13     
+ Misses       3242     3234       -8     

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