feat(weave): add display names to GenAI spans

[rgao-coreweave]

Description

Add first-class display names to the TypeScript GenAI span API.

• Add displayName to the shared span initialization options used by Turn, LLM, Tool, and SubAgent.
• Add a chainable setDisplayName method for updating an open span.
• Keep the weave.display_name wire attribute inside the SDK so integrations do not need to set the reserved key directly.
• Keep integration-specific display-name formatting outside the SDK.

This is additive and has no breaking changes. It supports follow-up adoption in wandb/weave-claude-code#147.

Testing

• pnpm test --runInBand — 487 tests passed
• pnpm exec jest --runInBand src/tests/genai/tool.test.ts src/tests/genai/turn.test.ts src/tests/genai/llm.test.ts src/tests/genai/subagent.test.ts — 51 tests passed
• pnpm run build
• pnpm run typecheck:cjs
• pnpm run typecheck:esm
• pnpm run lint
• pnpm run prettier-check