[OTel] Smaller cleanups

[krisztianfekete]

A few low-risk papercuts, each independent, please file PRs one-by-one:

• is_llm_span only counts a span if gen_ai.request.model is set; also classify by gen_ai.operation.name
• unknown trace format silently defaults to ADK; return an explicit "unrecognized" result instead
• gen_ai.system.message events are dropped everywhere, so system instructions never reach spans or the UI
• timestamp-unit guessing in incremental_processor.py produces garbage for microsecond inputs
• invocation dedup by exact user-text equality collapses legitimately identical prompts