fix: improve ProviderModelNotFoundError with suggestion for OpenRouter models

[konard]

Summary

This PR fixes issue #135 where users get a confusing ProviderModelNotFoundError when using incorrect model format for OpenRouter models.

Problem

User tried to use agent --model z-ai/glm-4.7 but the correct format is agent --model openrouter/z-ai/glm-4.7. The error message didn't provide any guidance on the correct format.

Root Cause

The parseModel() function splits on /, so z-ai/glm-4.7 is parsed as:

• providerID: z-ai
• modelID: glm-4.7

Since there's no provider named z-ai (the provider is openrouter), a ProviderModelNotFoundError is thrown.

Solution

Improved Error Messages: When a provider is not found, the code now checks if the model exists in another provider and adds a helpful suggestion to the error output.

Before: {"type":"error","message":"ProviderModelNotFoundError"}
After:  {"type":"error","message":"ProviderModelNotFoundError","hint":"Did you mean: openrouter/z-ai/glm-4.7?"}

Files Changed

• js/src/provider/provider.ts - Added suggestion field to error and model lookup logic
• js/src/index.js - Pass suggestion as hint in error output
• js/.changeset/improve-provider-error-messages.md - Changeset for this fix

Note: OpenRouter documentation was already added to the repository in the main branch.

Testing

• Lint checks pass
• Format checks pass
• File size checks pass
• Changes compile correctly
• Changeset added

Correct Usage

# Wrong (will give helpful suggestion now)
agent --model z-ai/glm-4.7

# Correct
agent --model openrouter/z-ai/glm-4.7

Fixes #135

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[konard]

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[konard]

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[konard]

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