fix(server-utils): Instrument LangGraph stream executions

[psh4607]

LangGraph CompiledGraph.stream() now creates the same gen_ai.invoke_agent parent span as invoke() and keeps it active until stream consumption finishes. The shared wrapper is used by manual server-runtime instrumentation and diagnostics-channel auto-instrumentation.

Root cause

The compile and create-agent wrappers only patched invoke(). stream() returned an async iterable without an agent span, so callback-created child spans became roots. Ending a span when stream() resolves would also be too early because graph work continues while the stream is consumed.

The implementation preserves the original stream object, observes async-iterator and ReadableStream consumption paths, forwards chunks unchanged, records accumulated response attributes when enabled, and ends the span on completion, cancellation, or error. It also suppresses LangGraph's internal invoke() to stream() call per compiled graph so existing invoke instrumentation does not emit duplicate agent spans.

JS-1858

Fixes #19626

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 794a336. Configure here.}