Python: Add experimental decorator to all Evals pieces

[Copilot]

Motivation and Context

All public Evals APIs are experimental and should carry the @experimental decorator to emit ExperimentalWarning on use, update docstrings with the stability notice, and set __feature_stage__ metadata — consistent with the existing pattern used for Skills.

Description

• _feature_stage.py: Added EVALS = "EVALS" to ExperimentalFeature enum.

• _evaluation.py: Imported ExperimentalFeature / experimental and applied @experimental(feature_id=ExperimentalFeature.EVALS) to all 19 public symbols:

  • Classes: EvalNotPassedError, ConversationSplitter, ConversationSplit, ExpectedToolCall, EvalItem, EvalScoreResult, EvalItemResult, EvalResults, Evaluator, AgentEvalConverter, CheckResult, LocalEvaluator
  • Functions: keyword_check, tool_called_check, tool_calls_present, tool_call_args_match, evaluator, evaluate_agent, evaluate_workflow

• _foundry_evals.py: Applied @experimental(feature_id=ExperimentalFeature.EVALS) to FoundryEvals, evaluate_traces, and evaluate_foundry_target.

On first use, callers will see:

ExperimentalWarning: [EVALS] LocalEvaluator is experimental and may change or be removed in future versions without notice.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? No.

[Copilot]

Pull request overview

Adds feature-stage metadata and runtime ExperimentalWarning behavior across the Python Evals API surface so that all public Evals entry points are consistently marked experimental (matching the existing Skills pattern).

Changes:

• Added EVALS to the ExperimentalFeature enum for feature-id tagging.
• Decorated core eval types and orchestration functions in _evaluation.py with @experimental(feature_id=ExperimentalFeature.EVALS).
• Decorated Foundry eval integration entry points (FoundryEvals, evaluate_traces, evaluate_foundry_target) with the same experimental feature id.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
python/packages/core/agent_framework/_feature_stage.py	Adds the EVALS experimental feature identifier used by decorators.
python/packages/core/agent_framework/_evaluation.py	Marks all public Evals core APIs as experimental via @experimental(...EVALS).
python/packages/foundry/agent_framework_foundry/_foundry_evals.py	Marks Foundry Evals public integration APIs as experimental via @experimental(...EVALS).