Add support for defining model as a step function initializer in DurableAgent

[TooTallNate]

Added support for defining model as a step function initializer in the AI workflow package.

What changed?

• Modified the DurableAgent class to accept either a string model ID or a function that returns a LanguageModelV2 instance
• Updated the doStreamStep function to handle both string model IDs and function initializers
• Added proper error handling for invalid model initialization arguments

How to test?

Test both initialization methods:

1. Using a string model ID:

const agent = new DurableAgent({
  model: 'anthropic/claude-opus',
  // other options
});

2. Using a step function initializer:

import { xai } from "@ai-sdk/xai"

const agent = new DurableAgent({
  model: async () => {
    "use step";
    return xai("grok-4");
  },
  // other options
});

Why make this change?

This enhancement provides more flexibility in how language models are initialized and used within the workflow. By supporting function initializers, users can now:

• Use custom model instances with specialized configurations
• Implement more complex model initialization logic that wasn't possible with just string identifiers

[changeset-bot]

🦋 Changeset detected

Latest commit: c9da9c4

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 10 packages

Name	Type
@workflow/ai	Patch
@workflow/swc-plugin	Patch
@workflow/builders	Patch
@workflow/cli	Patch
@workflow/next	Patch
@workflow/nitro	Patch
@workflow/sveltekit	Patch
workflow	Patch
@workflow/world-testing	Patch
@workflow/nuxt	Patch

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example-workflow	Ready	Preview	Comment	Nov 16, 2025 3:25am
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workbench-nitro-workflow	Ready	Preview	Comment	Nov 16, 2025 3:25am
workbench-nuxt-workflow	Ready	Preview	Comment	Nov 16, 2025 3:25am
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workbench-vite-workflow	Ready	Preview	Comment	Nov 16, 2025 3:25am
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[TooTallNate]

• [swc-plugin-workflow] Fix compiler warnings #348 : 2 dependent PRs (#349 , #350 )
• Add support for defining model as a step function initializer in DurableAgent #346 👈 (View in Graphite)
• Add support for step functions nested inside of workflow functions #345
• Add support for step functions defined as object properties #342
• Use "stepId" instead of Symbol.for("STEP_FUNCTION_NAME_SYMBOL") for annotating step functions #341
• Remove step function identifier transform out of swc-plugin and into useStep() runtime function #337
• Transform step functions to single useStep() calls #336
• main

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[VaguelySerious]

This PR seems to also contain changes from a different one changing SWC stuff - approving the DurableAgent changes specifically