feat(reporters): group GitHub summary failures by exception type

[thomhurst]

Summary

When multiple tests fail with the same exception type (e.g., 12 tests all throwing NullReferenceException from a shared setup method), the GitHub step summary currently shows them as 12 separate rows with nearly identical error messages.

Proposal

Group failures by exception type, showing a count header per group with a shared error message and a collapsible list of affected tests:

### Failures by Cause

<details>
<summary>NullReferenceException (12 tests)</summary>

| Test | Duration |
| --- | --- |
| MyService.Calculate(1, 2) | 0.03s |
| MyService.Calculate(3, 4) | 0.02s |
| ... |

**Common error:**
<pre>Object reference not set to an instance of an object.
   at MyApp.MyService.Init() in MyService.cs:line 42</pre>

</details>

Benefits

• Instantly answers "is this one bug or many?"
• Reduces summary size when bulk failures share a root cause
• The exception type is already available via FailedTestNodeStateProperty.Exception