docs: expand Vercel AI SDK migration guide with full option + v6 coverage

- Add exhaustive streamText -> chat() option mapping table covering
  every AI SDK v6 parameter (tools, toolChoice, activeTools, stopWhen,
  prepareStep, experimental_transform/context/telemetry/repairToolCall,
  all sampling controls, abort, headers, providerOptions -> modelOptions)
- Add streamText result -> TanStack equivalent table (textStream,
  fullStream, text, usage, finishReason, steps, toUIMessageStreamResponse,
  pipeTextStreamToResponse, consumeStream, etc.)
- Expand Generation Options with topK/presence/frequency/seed/stop under
  modelOptions, clarify flat typed modelOptions vs provider-keyed
  providerOptions
- New section: Structured Output (generateObject / streamObject / v6
  Output.object) -> outputSchema on chat(); notes on Standard Schema
  libraries, provider strategies, and the current gap for partial
  object streaming
- New section: Agent Loop Control — stopWhen / hasToolCall / stepCountIs
  mapped to maxIterations / untilFinishReason / combineStrategies, and
  prepareStep mapped to middleware onConfig/onIteration
- New section: Middleware — wrapLanguageModel + experimental_transform
  mapped to a single ChatMiddleware array; full hook inventory;
  toolCacheMiddleware usage; common-pattern mapping table
- New section: Observability — where to plug logging/metrics/tracing
- Update generateText coverage to chat({ stream: false }) returning a
  real Promise<string> (not just streamToText)
- Update Tool Approval "Before" to show AI SDK v6's native needsApproval
  + sendAutomaticallyWhen flow; the two APIs are now symmetric
- Reframe "Removed Features" -> "Features Not Yet Covered" and scope
  it to embeddings, partial-object streaming, built-in retries/timeouts
- Update frontmatter for the docs/migration/ location (order, description,
  keywords); fix cross-links to the new directory layout
  (../advanced/middleware, ../chat/structured-outputs, etc.)

Author: AlemTuzlak