fix(evaluation): preserve trace boundaries in eval sets

[linhongyu510]

Summary

Fix trace-to-eval-set conversion when a tracing file contains more than one trace.

Previously, conversion state such as conversation, app_name, and user_id was shared across trace groups, while eval_cases.append() ran after the grouping loop. This could collapse multiple traces into one eval case and mix tool calls or metadata across trace boundaries.

This change:

• creates one isolated eval case per trace
• sorts spans by start_time before deriving input, output, and tool order
• resets conversation and metadata state for every trace
• includes the trace ID in generated eval IDs
• derives the eval-set timestamp from the earliest generated case

The regression test uses two interleaved, out-of-order traces and verifies that conversations, tool calls, metadata, and timestamps remain isolated.

Validation

• .venv/bin/python -m pytest tests/test_evaluator.py -q — 3 passed
• ruff check veadk/evaluation/base_evaluator.py tests/test_evaluator.py
• ruff format --check veadk/evaluation/base_evaluator.py tests/test_evaluator.py
• git diff --check

AI assistance

This change was developed with AI assistance. I reviewed the trace grouping logic, rebased it onto the latest main, and ran the checks above.