fix(ai): prevent duplicate tool parts when model calls unavailable tool

[gdaybrice]

Summary

• Fixes a bug where calling an unavailable tool creates two UI parts with the same toolCallId — a static tool-{name} part and a dynamic-tool part
• The duplicate causes Bedrock (and potentially other providers) to reject the message with "duplicate Ids" when the message is sent back in conversation history
• Root cause: tool-input-start arrives without a dynamic flag (creating a static part), then tool-input-error arrives with dynamic: true (from parseToolCall's NoSuchToolError handling) and creates a new dynamic part instead of updating the existing one
• Fix: check for an existing part before deciding which update function to call, preserving the original part type

Reproduction

1. Configure an agent with a set of tools
2. Have the model call a tool name that isn't in the available tools (e.g., model hallucinates a tool name)
3. The resulting UI message will contain two parts with the same toolCallId — one tool-{name} and one dynamic-tool
4. On the next turn, convertToModelMessages converts both into tool-call content parts
5. The Bedrock provider's groupIntoBlocks merges them into one message, and Bedrock rejects it:

   The toolUse blocks at messages.54.content contain duplicate Ids: tooluse_Uel6zAsKe7Lve5LiXPGiyH