[langchain] Add nested workflow span on on_chain_start

[!NOTE]
This issue was generated with AI assistance and requires investigation and confirmation prior to being worked on.

Part of #543.

In on_chain_start, _looks_like_workflow currently suppresses child chains when parent_run_id is not None.

Semantic conventions specify that application-defined workflows should be reported even when nested (e.g. a sub-graph executed as a node of another graph).

• Map nested sub-graphs / workflows to GenAI invoke_workflow (gen_ai.operation.name: "invoke_workflow") with span name invoke_workflow {gen_ai.workflow.name}
• Capture gen_ai.workflow.name
• Capture gen_ai.input.messages and gen_ai.output.messages when content capture is enabled

Start in the on_chain_start entry point and inspect how _looks_like_workflow handles runs with a parent_run_id. Confirm the applicable semantic conventions, then verify that nested workflows produce invoke_workflow spans with the workflow name and capture input and output messages when content capture is enabled.