Update skip_reason to clarify cancelled leg scenario (#127479)

## Description

Improves the `skip_reason` message set on pipelines with zero test
results to hint at the most common root cause: a cancelled leg.

### Changes

- **`extract_failed_tests.py`**: Updated `skip_reason` string from
`'Build failed but no test failures detected via Test Results API'` to
`'Build failed but no test failures detected via Test Results API, e.g.,
due to a cancelled leg'`

<!-- -->

<!-- START COPILOT ORIGINAL PROMPT -->



<details>

<summary>Original prompt</summary>

> Update the `skip_reason` string on line 267 of
`.github/skills/ci-pipeline-monitor/scripts/extract_failed_tests.py` in
the `dotnet/runtime` repository.
> 
> Change the existing string from:
> 
> ```
> Build failed but no test failures detected via Test Results API
> ```
> 
> to the exact string:
> 
> ```
> Build failed but no test failures detected via Test Results API, e.g.,
due to a cancelled leg
> ```
> 
> The line currently reads:
> 
> ```python
> "UPDATE pipelines SET result = 'inconclusive', skip_reason = 'Build
failed but no test failures detected via Test Results API' WHERE name =
?",
> ```
> 
> After the change, it should read:
> 
> ```python
> "UPDATE pipelines SET result = 'inconclusive', skip_reason = 'Build
failed but no test failures detected via Test Results API, e.g., due to
a cancelled leg' WHERE name = ?",
> ```
> 
> Only this single line needs to be modified. Do not change any other
occurrences or files unless an identical duplicate of this exact string
exists elsewhere in the same script and clearly needs to be kept
consistent.
> 
> The following is the prior conversation context from the user's chat
exploration (may be truncated):
> 
> User: File a PR to change this line to set skip_reason to Build failed
but no test failures detected via Test Results API, e.g., due to a
> cancelled leg.
> 
> Assistant: Waiting for your response to the dialog before proceeding
with creating the pull request.
> 
> User: @Copilot Dismissed Confirmation: Confirm agent session
> 
> Assistant: Understood — I won't create the PR. Let me know if you'd
like me to proceed with it later.
> 
> User: It should be exact string "Build failed but no test failures
detected via Test Results API, e.g., due to a cancelled leg"
> 
> Assistant: I've started a coding agent session to open the PR updating
the `skip_reason` string to `"Build failed but no test failures detected
via Test Results API, e.g., due to a cancelled leg"`.
> 
> 


</details>



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*This pull request was created from Copilot chat.*
>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: JulieLeeMSFT <63486087+JulieLeeMSFT@users.noreply.github.com>

Author: Copilot

.github/skills/ci-pipeline-monitor/scripts/extract_failed_tests.py

@@ -264,7 +264,7 @@ def main():
     # Mark pipelines with 0 test results as inconclusive
     for name in zero_result_pipelines:
         conn.execute(
-            "UPDATE pipelines SET result = 'inconclusive', skip_reason = 'Build failed but no test failures detected via Test Results API' WHERE name = ?",
+            "UPDATE pipelines SET result = 'inconclusive', skip_reason = 'Build failed but no test failures detected via Test Results API, e.g., due to a cancelled leg' WHERE name = ?",
             (name,)
         )
     if zero_result_pipelines: