[P1][Benchmark] BrowseComp agentic search benchmark integration tracking

[Luodian]

Problem Statement

Agentic search benchmark coverage is incomplete for current frontier comparisons. BrowseComp is frequently reported by model providers but not yet integrated as an lmms-eval task.

Proposed Solution

Add BrowseComp benchmark integration under lmms_eval/tasks/browsecomp/, including task config(s), tool-use or constrained interaction adapter as required by benchmark format, and aggregation metrics.

Acceptance Criteria

• lmms_eval/tasks/browsecomp/ exists with runnable YAML + utils.py.
• BrowseComp task names appear in python -m lmms_eval --tasks list.
• At least one smoke run (--limit 5) works with configured backend.
• docs/current_tasks.md documents BrowseComp support.

Implementation Plan

1. Map BrowseComp task format to lmms-eval output type.
2. Implement evaluation and scoring utilities.
3. Add task YAML and defaults for generation args.
4. Add docs entry and example invocation.

Technical Notes

• Align with existing generate_until_agentic and agentic trace pathways if benchmark requires tool steps.
• Keep deterministic mode available for CI-friendly smoke tests.

[Luodian]

Resolved via PR #1190 (merged). BrowseComp task added.