Validate Required Params for initialize and resources/subscribe

## Motivation and Context

Fixes https://github.com/modelcontextprotocol/ruby-sdk/issues/450

The MCP schema requires `protocolVersion`, `capabilities`, and `clientInfo` on `initialize` params,
and `uri` on `resources/subscribe` and `resources/unsubscribe` params. The server accepted requests with
these fields absent: `initialize` silently negotiated the server's default protocol version,
and `resources/subscribe` returned an empty success result. Both now return `-32602` Invalid params,
matching the Python SDK, which rejects such requests during pydantic validation.
The `uri` check runs before dispatch so it also covers handlers registered via `resources_subscribe_handler`
and `resources_unsubscribe_handler`.

A `protocolVersion` value that is present but unsupported still negotiates the server's fallback version,
as the spec's lifecycle section requires.

## How Has This Been Tested?

Reproduced the issue's stdio repro steps: both malformed requests now return `-32602` instead of succeeding.

## Breaking Changes

Clients that send `initialize` without `protocolVersion`, `capabilities`, or `clientInfo`,
or `resources/subscribe` / `resources/unsubscribe` without `uri`, now receive `-32602` Invalid params
instead of a success response. Such requests violate the MCP schema, but any code relying on
the previous lenient behavior (for example, test harnesses sending bare `initialize` requests)
must now include the required fields.

test/mcp/server/transports/stdio_transport_test.rb

@@ -108,6 +108,7 @@ module MCP
             id: "1",
             params: {
               protocolVersion: "2025-11-25",
+              capabilities: {},
               clientInfo: { name: "stdio-client", version: "1.0" },
             },
           }
@@ -140,6 +141,7 @@ module MCP
             id: "first",
             params: {
               protocolVersion: "2025-11-25",
+              capabilities: {},
               clientInfo: { name: "original", version: "1.0" },
             },
           }
@@ -149,6 +151,7 @@ module MCP
             id: "second",
             params: {
               protocolVersion: "2024-11-05",
+              capabilities: {},
               clientInfo: { name: "intruder", version: "9.9" },
             },
           }