Fix serialization errors in custom error handling

[moutonjeremy]

Enhance error handling in the custom error handler and Method function to manage serialization failures. Add tests to verify behavior when encountering unserializable outputs and errors.

[Copilot]

Pull request overview

This PR enhances error handling in the fiberoapi library to gracefully manage serialization failures when converting responses and errors to JSON. The changes ensure that when unserializable data structures (like channels) are returned, the API returns appropriate error responses instead of failing silently.

Key Changes:

• Added error handling for serialization failures in the Method function to return structured error responses
• Enhanced handleCustomError to catch and handle serialization failures of custom error objects
• Added comprehensive test coverage for both response and error serialization failures

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
serialization_error_test.go	New test file with two test cases verifying behavior when encountering unserializable outputs (channels) in both success responses and error responses
fiberoapi.go	Modified Method function to check for JSON serialization errors and return a structured ErrorResponse with type "serialization_error" when serialization fails
common.go	Enhanced handleCustomError function to catch serialization failures and return a fallback error response with a simple message

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