feat(otel): tracing helpers and header propagation - #910

⚠️ Restack in progress. Do not merge this PR. Replacement PR: #915.

Summary

• add optional OpenTelemetry helper for client tracing
• propagate trace headers through plot/upload/remote GFQL paths
• add focused tests for header propagation

Stack

• Base: master
• Stack note: perf(hop): traversal optimizations and fast path #911 was merged into pr-886-otel (this branch) and feat(gfql): add same-path WHERE executor #912 was merged into pr-886-hop-chain (branch on top of perf(hop): traversal optimizations and fast path #911). These are stack-merged, not in master yet.
• Parallel: chore(bench): add GFQL benchmarks and CI job #913 (benchmarks), Docs: GFQL WHERE reference #914 (docs on top of chore(bench): add GFQL benchmarks and CI job #913)

Testing

• uv run pytest graphistry/tests/test_arrow_uploader.py
• uv run pytest graphistry/tests/test_chain_remote_auth.py
• uv run pytest graphistry/tests/test_trace_headers_behavior.py