fix: Surface total SignalFx alert formatting failure

[mvanhorn]

Closes #

📑 Description

Keep the existing per-incident isolation so one malformed incident does not discard valid alerts from the same response. Track whether formatting failures occurred and, after processing, raise a descriptive exception only when the upstream response was non-empty and no incident could be converted; preserve the original formatting failure as the cause where practical. Continue returning [] for a genuinely empty response and returning the successful subset for mixed valid/invalid responses.

SignalfxProvider._get_alerts catches and logs each incident-formatting exception, then returns the successfully formatted alerts. This preserves useful partial results, but when a non-empty API response contains only malformed incidents it returns [], making total data loss indistinguishable from a genuinely empty SignalFx response. Transport errors already propagate through raise_for_status, so the defect is limited to the per-incident formatting boundary. The base provider's get_alerts method already delegates to this method and will propagate its exception, so no caller or registry change is required.

Closes #6674

✅ Checks

• My pull request adheres to the code style of this project
• My code requires changes to the documentation
• All the tests have passed

ℹ Additional Information

Nothing beyond what is described above.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 30.46%. Comparing base (ce302f6) to head (9af3294).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6684       +/-   ##
===========================================
- Coverage   46.42%   30.46%   -15.96%     
===========================================
  Files         178      101       -77     
  Lines       18694    11773     -6921     
===========================================
- Hits         8679     3587     -5092     
+ Misses      10015     8186     -1829     

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