fix: avoid host-only eval crash when app_agent_log is empty

[grtninja]

Summary

• guard host-only evaluation paths so app_agent_log=[] no longer crashes user_content_construction_head_tail()
• keep the existing initial-screenshot behavior when an AppAgent step is present
• add focused regression coverage for empty and non-empty AppAgent logs

Why

Fixes #304. Host-only tasks can complete without any AppAgent steps, so indexing trajectory.app_agent_log[0] is unsafe and can terminate evaluation with IndexError.

Validation

• python -m pytest -q tests\\unit\\test_eva_prompter_host_only.py

Risk Notes

• small evaluation-path change only
• no automation-flow refactor
• no policy or security-surface change

[grtninja]

@microsoft-github-policy-service agree

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