Fix: return protocol errors for invalid arguments and server errors

[sqrlington]

Motivation and Context

PR #231 fixed unknown tool calls to return protocol errors per the MCP spec, but two of the three protocol error cases still return isError: true instead of JSON-RPC errors:

• Invalid arguments — missing required arguments and schema validation failures return isError: true instead of -32602
• Server errors — uncaught exceptions during tool execution return isError: true instead of -32603

Per the MCP spec, these are protocol errors, not tool execution errors.

Fixes #284.

How Has This Been Tested?

Updated existing tests to assert JSON-RPC error responses instead of isError: true results. All tests pass, rubocop clean.

Breaking Changes

Clients relying on isError: true for invalid arguments or server errors will now receive JSON-RPC error responses instead. This matches the behavior PR #231 introduced for unknown tools.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

[koic]

call_tool no longer calls report_exception before re-raising. The rescue here now receives RequestHandlerError instead of the original exception, so exception_reporter loses the original
class and backtrace.

--- a/lib/mcp/server.rb
+++ b/lib/mcp/server.rb
@@ -317,7 +317,8 @@ module MCP

           result
         rescue => e
-          report_exception(e, { request: request })
+          reported = e.is_a?(RequestHandlerError) ? (e.original_error || e) : e
+          report_exception(reported, { request: request })
           if e.is_a?(RequestHandlerError)
             add_instrumentation_data(error: e.error_type)
             raise e

[sqrlington]

Took the liberty of refactoring the rescue block to avoid the duplicated is_a? checks when I implemented this suggestion

[koic]

Ah, great! Thanks!