Surface empty BCPT results as a warning instead of silent success (#2229)

## Problem

When BCPT performance tests run but produce no log entries (for example,
because the test runner exits early, the suite codeunit IDs do not match
the published app, or the job fails silently), the results file contains
no entries. \GetBcptSummaryMD\ returned an empty string, the
\AnalyzeTests\ step wrote nothing to the job summary, and the build
succeeded with no indication that performance tests did not produce
output.

Fixes #2158

## Changes

### Actions/AnalyzeTests/TestResultAnalyzer.ps1

- \GetBcptSummaryMD\: Distinguish between the results file not existing
(return empty string, no test was run) and the results file existing but
containing no entries (return an informational message). The message is
written to the GitHub step summary under the Performance test results
heading, making the empty result visible.

### Actions/AnalyzeTests/AnalyzeTests.ps1

- After calling \GetBcptSummaryMD\, check whether the results file is
present but the summary is still empty. Emit a \::Warning::\ annotation
in that case as a defence-in-depth measure.

### Tests/AnalyzeTests.Test.ps1

- Add tests for \ReadBcptFile\ when the file does not exist and when it
exists but contains no entries.
- Add a test for \GetBcptSummaryMD\ verifying that it returns an empty
string when the file does not exist and a non-empty warning message when
the file exists but contains no entries.

## Testing

1. Create a BCPT test app and configure a suite with codeunit IDs that
do not match any published app.
2. Run the AL-Go pipeline so that \RunBCPTTests.ps1\ exits without
recording measurements.
3. Before this fix: the Performance test results step writes nothing to
the summary and the build succeeds silently.
4. After this fix: the summary shows a message stating that no BCPT
results were recorded, and a warning annotation appears on the step.

You can also run the Pester test suite locally:
\\\powershell
Invoke-Pester Tests/AnalyzeTests.Test.ps1
\\\
All 10 tests should pass.

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Alexander Holstrup <117829001+aholstrup1@users.noreply.github.com>