[Fix] Combine split eval results in default summarizer

[yhzhu99]

Summary

• combine numbered split result files in the default summarizer when the unsplit result file is absent
• weight merged numeric metrics by sample counts from result metadata or legacy details
• record per-result sample counts from OpenICLEvalTask for future split runs without details

Fixes #2163

Tests

• python3 -m flake8 tests/summarizers/test_default.py opencompass/tasks/openicl_eval.py opencompass/utils/result.py
• python3 -m py_compile opencompass/summarizers/default.py opencompass/tasks/openicl_eval.py opencompass/utils/result.py tests/summarizers/test_default.py
• python3 -m pytest tests/summarizers/test_default.py (blocked locally: missing torch runtime dependency)