test: add unit tests for FrameTracer and DivergenceDetector (zero coverage)

[acailic]

Problem

agent_debugger_sdk/core/frame_tracer.py (603 lines) and agent_debugger_sdk/core/divergence_detector.py (730 lines) have zero test coverage.

These are substantial modules implementing:

• FrameTracer: function-level tracing, frame lifetime capture, call tree building, cost analysis (ADI / FSE 2026)
• DivergenceDetector: structural, temporal, behavioral, state, and performance divergence detection between sessions

Every other research module (ConformalScorer, CausalTracer, ErrorAttribution, SafetyMonitor, RedundancyScorer) has tests in tests/test_research_features.py. These two do not.

Scope

Add a new test file tests/test_frame_tracer_divergence.py (or extend test_research_features.py) covering:

FrameTracer

• TokenUsage arithmetic and serialization
• FrameEvent dataclass and to_dict()
• FrameLifetimeTrace construction
• FrameTracer.trace() decorator/context manager
• Call tree building and cost breakdown

DivergenceDetector

• DivergenceType / DivergenceSeverity enum values
• DivergencePoint dataclass and to_dict()
• detect_divergences() with matching and diverging event lists
• compare_session_structures()
• analyze_temporal_divergence()
• analyze_behavioral_divergence()

Validation

.venv-ci/bin/python3 -m pytest -q tests/test_frame_tracer_divergence.py
.venv-ci/bin/ruff check .

Labels

• enhancement, test