Standardize Resource Not Found Errors on -32602 with URI Data per SEP-2164

[koic]

Motivation and Context

SEP-2164 (modelcontextprotocol/modelcontextprotocol#2164, merged for the 2026-07-28 spec release) standardizes the resource-not-found error on the JSON-RPC Invalid Params code (-32602), dropping the legacy -32002 code, and both reference SDK implementations attach the requested URI as structured error data (typescript-sdk#2267 throws InvalidParams with data: { uri }; python-sdk#2344 adds a ResourceNotFoundError that maps to code=-32602, data={"uri": ...}).

The Ruby server already used -32602 (via error_type: :invalid_params) for its built-in not-found path (completion/complete with an unknown ref/resource URI), but the response carried no structured data and the generic "Invalid params" message, and there was no public error class for resources_read_handler blocks to raise. The Ruby client never special-cased -32002, so nothing needed removal there.

• New public MCP::Server::ResourceNotFoundError (modeled on the URLElicitationRequiredError precedent): code -32602, message "Resource not found: <uri>", and data: { uri: <uri> }. Custom resources_read_handler blocks raise it for unknown URIs; the README documents the pattern.
• The completion/complete ref/resource not-found path now raises this class, so its error response gains data: { uri: } and a descriptive message instead of the generic "Invalid params" text.
• The default resources/read behavior (returning empty contents for unknown URIs when no handler is registered) is intentionally unchanged; raising there would alter existing wire behavior.

Resolves #379.

How Has This Been Tested?

New tests in test/mcp/server_test.rb:

• resources/read with a handler raising ResourceNotFoundError responds with code -32602, message "Resource not found: file:///missing.txt", and data: { uri: "file:///missing.txt" }
• completion/complete with an unknown ref/resource URI carries the same code, message, and structured data

New test in test/mcp/client_test.rb asserts Client#read_resource surfaces the -32602 code and the data hash unmodified through Client::ServerError (regression: no special-casing swallows it).

The pre-existing completion error-code tests pass unchanged. bundle exec rake (tests, RuboCop, and conformance baseline) passes.

Breaking Changes

None at the JSON-RPC code level (-32602 before and after). The completion/complete resource-not-found error response changes shape within the error object: message becomes "Resource not found: <uri>" instead of "Invalid params", and data becomes { uri: <uri> } instead of the message string. Both members are advisory per JSON-RPC 2.0.

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