fix(otel): add GenAI fallbacks to direct extraction helpers

[jakepresent]

Summary

• Adds OTel GenAI (gen_ai.*) fallbacks to direct extraction helpers: span, trajectory, and session extraction.
• Makes SpanNode.to_dict() / TREE and CHUNKED judge extraction serialize GenAI model metadata, messages, tool spans, and event/message-carried tool calls instead of leaving fields blank.
• Preserves OpenInference precedence for dual-emitting spans and keeps existing OpenInference truncation behavior for raw tool args.

Fixes #241

Tests

• python -m py_compile assert_ai/core/otel.py tests/test_otel_genai.py
• python -m pytest tests/test_otel_genai.py::TestGenAIDirectExtractionFollowup tests/test_otel_genai.py tests/test_framework_agnostic.py -q — 162 passed, 1 skipped
• python -m pytest tests/ -q — first run hit a transient heartbeat timing failure in tests/test_runtime_safety.py::test_heartbeat_writes_manifest_periodically; targeted rerun passed
• python -m pytest tests/ -q — 1091 passed, 19 skipped, 1 warning, 474 subtests passed

Note: the fresh worktree has no viewer/node_modules, so the full-suite run temporarily symlinked viewer/node_modules from the main checkout and removed the symlink afterward.

[AaronAspinwall123]

@jakepresent looks stale, please update