Move langchain.chain.name to gen_ai.pipeline.name

[RulaKhaled]

Follow-up to #23582. langchain.chain.name is a JS-only attribute with no entry in the conventions registry, while gen_ai.pipeline.name — "Name of the AI pipeline or chain being executed" — covers exactly this value and is already set by our LangGraph instrumentation (packages/server-utils/src/ai/langgraph/index.ts:115). LangChain is the odd one out.

Conventions

• Register langchain.chain.name as deprecated, with gen_ai.pipeline.name as its replacement
• Add {{gen_ai.operation.name}} {{gen_ai.pipeline.name}} to the gen_ai agent name templates, so invoke_agent format_prompt is template-backed

SDK

• Emit gen_ai.pipeline.name instead of langchain.chain.name on LangChain chain spans (packages/server-utils/src/ai/langchain/index.ts, handleChainStart)
• Skip the attribute entirely when the chain is unnamed, rather than writing the unknown_chain sentinel into a registered attribute — matches LangGraph, which only sets pipeline.name when a graph name exists
• Update unit + integration tests that assert langchain.chain.name
• Add a MIGRATION.md bullet under the existing gen_ai attribute renames in the v11 section

Notes

• Breaking change: the attribute ships in v10, so dashboards/alerts/ignoreSpans rules keyed on langchain.chain.name will silently stop matching. Needs to land before v11 releases so it rides along with the renames already listed there.
• Span names are unaffected. feat(server-utils): Emit low-cardinality gen_ai agent span names when streaming #23582 builds invoke_agent {chainName} from a local variable, not from the attribute, so the rename doesn't change any name.

[linear-code]

JS-3452