feat(server): add Tool call error categories

[duwenxin99]

Create Agent vs Server error types to distinguish between the two types.

[gemini-code-assist]

Summary of Changes

Hello @duwenxin99, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a structured error handling mechanism by defining distinct error categories for agent and server-side issues. This foundational change aims to improve the clarity and diagnosability of errors within the system, making it easier to identify the source of a problem and streamline debugging efforts.

Highlights

• New Error Definitions File: A new file, internal/util/errors.go, has been added to centralize and define custom error types for the server.
• Error Categorization: Introduced an ErrorCategory enum with CategoryAgent and CategoryServer to explicitly classify the origin of errors.
• Custom Error Interface: Defined a ToolboxError interface that all custom errors must satisfy, ensuring a consistent error structure.
• Structured Error Types: Implemented AgentError and ServerError structs, both of which fulfill the error interface and provide a Category() method for type-specific error reporting. ServerError also includes a Cause field for wrapping underlying errors.

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[gemini-code-assist]

Code Review

This pull request introduces new error types, AgentError and ServerError, to categorize errors originating from the agent or the server. The changes are well-contained in a new errors.go file. My feedback includes suggestions to improve the new error abstractions by making the ToolboxError interface more specific and by adding support for Go's standard error wrapping to ServerError.