fix(ai): Don't create duplicate tool parts when models call non-existent tools

[josh-williams]

Background

When a model calls a tool that doesn't exist in the tools object (e.g. a hallucinated tool name), processUIMessageStream creates two message parts for the same toolCallId — one static (tool-{toolName}) and one dynamic (dynamic-tool). This causes downstream failures when the conversation is continued, since the model API rejects requests containing two tool results for the same tool call ID.

Summary

Fixes a bug in processUIMessageStream where a model calling a non-existent tool produces two UI message parts for the same toolCallId — one static (tool-{toolName}) and one dynamic (dynamic-tool). This happens because tool-input-start creates a static part (when dynamic is undefined for an unknown tool), but the subsequent tool-input-error arrives with dynamic: true and only searches for a dynamic-tool part, missing the existing static one and creating a duplicate. The fix checks for an existing part by toolCallId before branching on chunk.dynamic, so the error updates the existing part in place instead of creating a second one.

Manual Verification

Manually verified against my own application. After the fix, I got a single tool part like:

            {
                "type": "tool-dashboard-page-create-dashboard",
                "state": "output-error",
                "rawInput":
                {
                    "name": "Staging Monitoring"
                },
                "errorText": "Model tried to call unavailable tool 'dashboard-page-create-dashboard'. Available tools: ....",
                "toolCallId": "toolu_015E6Kd6cwSMkw7SFpsa7w88",
                "callProviderMetadata":
                {
                    "anthropic":
                    {
                        "caller":
                        {
                            "type": "direct"
                        }
                    }
                }
            },

Checklist

• Tests have been added / updated (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• A patch changeset for relevant packages has been added (for bug fixes / features - run pnpm changeset in the project root)
• I have reviewed this pull request (self-review)

Related Issues

Fixes #12772

[gr2m]

Looks good, thank you Josh!