fix: raise when all SignalFx incidents fail to format

[DragonBot00]

Problem

SignalFx _get_alerts silently returns [] when every incident fails to format. This masks errors — callers get an empty list with no indication that something went wrong. Other providers (VictoriaMetrics, Prometheus, Dynatrace) raise on total failure.

Fix

Track the total number of incidents and how many failed to format. If all incidents fail, raise an exception instead of silently returning an empty list. Partial failures still return successfully (matching the existing per-item skip pattern used by Datadog and others).

Fixes #6674

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 30.46%. Comparing base (ce302f6) to head (eecf425).
⚠️ Report is 22 commits behind head on main.

Files with missing lines	Patch %	Lines
keep/rulesengine/rulesengine.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6775       +/-   ##
===========================================
- Coverage   46.42%   30.46%   -15.97%     
===========================================
  Files         178      101       -77     
  Lines       18694    11775     -6919     
===========================================
- Hits         8679     3587     -5092     
+ Misses      10015     8188     -1827     

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