feat(ai): add preparedTools/preparedToolChoice to experimental_streamModelCall

[gr2m]

Summary

Add support for pre-prepared serializable tool descriptors in experimental_streamModelCall. When preparedTools is provided, the function skips prepareToolsAndToolChoice and uses the descriptors directly. This is needed to unblock #13820.

Alternative approach: #13824

Problem

experimental_streamModelCall requires ToolSet for its tools parameter. ToolSet contains Zod schemas and execute functions — neither of which are serializable. When streamModelCall is called inside a Workflow 'use step' function, the ToolSet crosses the step serialization boundary and fails:

[Workflow] Serialization failed {
  context: 'step arguments',
  problematicValue: ZodObject { ... }
}

Solution

Allow callers to pre-prepare tools outside the serialization boundary and pass the serializable result through:

// Outside the step boundary:
const { tools: preparedTools, toolChoice: preparedToolChoice } =
  await experimental_prepareToolsAndToolChoice({ tools, toolChoice });

// Inside the step (serialization boundary):
const { stream } = await experimental_streamModelCall({
  model,
  messages,
  preparedTools,      // JSON schemas, no Zod
  preparedToolChoice, // plain object
});

New types

Intentionally alias internal LanguageModelV4* types to shield consumers from provider spec version changes:

• FunctionToolDescriptor — serializable function tool descriptor (JSON schema, name, description)
• ProviderToolDescriptor — serializable provider tool descriptor
• ToolDescriptor — union of the above
• ToolChoiceDescriptor — serializable tool choice (auto | none | required | { tool: name })

New exports from ai

• FunctionToolDescriptor, ProviderToolDescriptor, ToolDescriptor, ToolChoiceDescriptor (types)
• experimental_prepareToolsAndToolChoice (function — extracts serializable descriptors from ToolSet)

Test plan

• pnpm build in packages/ai — passes
• pnpm test:node in packages/ai — 2266 tests pass
• Existing tests unaffected (preparedTools is optional, no behavior change when not provided)

[gr2m]

Alternative approach: #13824 removes tools: ToolSet from streamModelCall entirely, accepting only ToolDescriptor[]. Cleaner API but breaking change — all callers must call prepareToolsAndToolChoice first.