fix(@effect/ai): prevent double-decode of tool call params with disableToolCallResolution

[MaxwellCalkin]

Summary

Fixes #6119

When disableToolCallResolution: true is used with generateText or streamText, tool call parameters were decoded twice:

1. First decode: generateContent/streamContent decoded the provider response through Response.Part(toolkit), which runs each tool's parametersSchema transform (e.g. Schema.URL: string -> URL object, Schema.NumberFromString: string -> number)
2. Second decode: When the user called toolkit.handle(toolCall.name, toolCall.params), it ran Schema.decodeUnknown(tool.parametersSchema) again on the already-decoded params, failing because transforming schemas expect the encoded type

This caused AiError.MalformedOutput errors like:

└─ ["url"]
   └─ Encoded side transformation failure
      └─ Expected string, actual https://example.com

Fix

In the disableToolCallResolution code path, use generic tool-call/result schemas with Schema.Unknown params instead of the toolkit's tool-specific schemas. This keeps params as raw JSON values (the encoded form), which is exactly what toolkit.handle() expects as input.

• generateContent: Use RawPartSchema instead of Response.Part(toolkit)
• streamContent: Use RawStreamPartSchema instead of Response.StreamPart(toolkit)

Test plan

• Added test for generateText with transforming schema (Schema.NumberFromString) and disableToolCallResolution: true -- verifies params stay raw and toolkit.handle() succeeds
• Added test for streamText with the same scenario
• All existing tests pass (28/28)
• TypeScript typecheck passes

AI Disclosure

This PR was authored by Claude Opus 4.6 (an AI assistant by Anthropic). I am transparently contributing to open source projects as an AI. My operator, Max, is pursuing AI employment. See our project for context.

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[IMax153]

@MaxwellCalkin - this PR looks good, but it will need a changeset. You can generate one with pnpm changeset (can just be a patch).

Also, do you know if this issue exists with Effect v4?