v4 #1841

Collection issue of things that we may or may not want to do for v4:

• Flatten Eval() types as much as possible (ref!: Simplify Eval type definitions #2516)
• Remove exports for plugins (plugins == diagnostics channels consumers) (ref!: Remove plugins as part of public API #2386)
• Clean up exports in general (don't export anything that is 100% necessary; reduce exported types/interfaces to an absolute minimum - [ ] exported types that are simultaneously function inputs cannot be widened because someone may have done const a: SomeOfOurType = { ... }) (ref!: Reduce API to a minimum that makes sense #2429)
• Add node engine requirements to package.json (feat(v4)!: Add Node Engine to package json #2378)
• Remove js braintrust cli (ref(v4)!: Remove JS Braintrust CLI #2377)
• Remove flue pre-v1 support (ref(v4)!: Remove flue pre-v1 support #2380)
• remove eve pre 0.34.0 support (added events) (ref(eve)!: Bump minimum eve version to 0.34.0 #2385)
• Remove unsubscribe functionality. It's barely used and a tad dangerous. (ref!: Remove plugins as part of public API #2386)
• Don't expose schema from SDK (ref!: Don't make generated types part of public api #2517)
• remove span flushAsync shenanigans (ref!: Remove flushAsync option #2409)
• Remove AI SDK v3 support (it accounts for 1.5% of downloads) (ref(ai-sdk)!: Remove AI SDK v3 support #2474)
• Remove deprecated packages (chore!: Remove deprecated packages (langchain, openai agents, ai sdk) #2485)
• Guard all multimodal stuff behind BRAINTRUST_CAPTURE_ATTACHMENTS (feat!: Add BRAINTRUST_CAPTURE_ATTACHMENTS env var option and captureAttachments logger option for all integrations #2519)
• Remove tools and parameters? (refactor(sdk)!: Remove deprecated tool APIs #2609)
• Make build setup super clean for tree shaking and such
• Rework BraintrustState -> Likely some form of client that is stateful and can be logged in or not logged in. Also give client an export. Probably too much for this major. We'll do a separate one for just this change.