evals metadata in builtin JUnit reporter

[giovanni-guidini]

These changes annotate the testTask in a way that the built-in JUnit reporter can expose the evals metadata - similar to what is done for the JSON reporter.

The formatting is discussed at length here JUnit is a very pervasive format, well established in the industry.

As an example, generating the JUnit report for the tests in the repo we get for one of the tests:

<testcase classname="src/ai-sdk-integration.test.ts" name="Tool Argument Validation &gt; What&apos;s the weather in Seattle in Celsius?" time="0.004360291">
  <properties>
      <property name="evals.scores.score_0.value" value="1">
      </property>
      <property name="evals.scores.score_0.type" value="float">
      </property>
      <property name="evals.scores.score_0.metadata.rationale" value="All expected tools were called">
      </property>
      <property name="evals.toolCalls.0.id" value="call_9999">
      </property>
      <property name="evals.toolCalls.0.name" value="getWeather">
      </property>
      <property name="evals.toolCalls.0.arguments.location" value="Seattle">
      </property>
      <property name="evals.toolCalls.0.arguments.units" value="celsius">
      </property>
      <property name="evals.toolCalls.0.result.temperature" value="18">
      </property>
      <property name="evals.toolCalls.0.result.condition" value="partly cloudy">
      </property>
      <property name="evals.toolCalls.0.status" value="completed">
      </property>
      <property name="evals.toolCalls.0.type" value="function">
      </property>
  </properties>
</testcase>

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.43%. Comparing base (c0945ef) to head (22b06c7).
⚠️ Report is 19 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #22      +/-   ##
==========================================
+ Coverage   84.50%   86.43%   +1.93%     
==========================================
  Files           4        4              
  Lines         400      457      +57     
  Branches      115      134      +19     
==========================================
+ Hits          338      395      +57     
  Misses         62       62              

Flag	Coverage Δ
unittests	86.43% <100.00%> (+1.93%)	⬆️

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[dcramer]

should we consider just using the term model here?

aside I think metadata might be entirely arbitrary in scorers