Validate Required Params for resources/read

[yuki3738]

Motivation and Context

The MCP schema requires a string uri in resources/read params. The server dispatched the request without checking it, and the outcome depended on how the read was served. A custom resources_read_handler received the params unchanged, so a handler that echoes params[:uri] answered with a success result carrying null. The class-based router raised a TypeError on a non-string uri once a resource template was registered, which surfaced as -32603 Internal error. Both now return -32602 Invalid params, matching the check modelcontextprotocol#451 added for initialize and resources/subscribe. The check runs before dispatch, so it covers custom handlers and the class-based router alike.

This change validates that uri is present and is a string. It does not validate URI syntax: that needs a separate change, weighing what to reject against the risk to existing clients.

Before, with a resources_read_handler that echoes params[:uri]:

params: {}          -> {"result":{"contents":[{"uri":null,...}]}}
params: {"uri":123} -> {"result":{"contents":[{"uri":123,...}]}}
no params           -> {"error":{"code":-32603,"message":"Internal error",...}}

Before, with a class-based resource template registered:

params: {"uri":123} -> {"error":{"code":-32603,"message":"Internal error",...}}

After, for every case above:

-> {"error":{"code":-32602,"message":"Invalid params","data":"Missing or invalid uri"}}

How Has This Been Tested?

Added tests for params absent, params that are not an object, uri absent, and a non-string uri. The non-string case is covered with a class-based resource and with a class-based template registered, since only the template path reached the TypeError. A further test asserts that a custom resources_read_handler never runs when uri is missing. Also verified by hand over Server#handle_json that each of these requests now answers -32602. rake test passes (1784 runs, 0 failures).

Breaking Changes

A resources/read request without uri, or with a non-string uri, now receives -32602 Invalid params. Earlier releases passed such a request to the handler, which could answer it successfully, or raised -32603 Internal error from the class-based router. Such requests violate the MCP schema, and a valid string uri is unaffected.

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