Align modern envelope validation with the finalized spec

## Motivation and Context

Two deviations from the finalized 2026-07-28 specification in the SEP-2575 envelope validation,
both introduced against the frozen SEP text and overtaken by post-final spec changes:

1. `clientInfo` was treated as required. Spec PR modelcontextprotocol/modelcontextprotocol#3002 made it
optional (clients SHOULD include it unless configured not to), and the TypeScript and Python SDKs validate only
the `protocolVersion` + `clientCapabilities` pair.
A conformant client configured not to identify itself was rejected.

2. A missing or mistyped envelope answered `-32600` Invalid Request.
The spec maps missing required envelope fields to `-32602` Invalid params, and both reference SDKs answer
with `-32602` naming the offending keys.

Fixing these also corrects the era classification to match the reference SDKs: a request claims
the modern lifecycle when `_meta` carries `io.modelcontextprotocol/protocolVersion`
(the TypeScript envelope claim and the Python `_has_modern_envelope` are both single-key checks),
and a claimed-but-incomplete envelope is now validated and rejected with `-32602` naming
the missing keys instead of silently flowing through the legacy path. Legacy `_meta` usage without
the claim key (`progressToken`, trace context) is classified exactly as before.

Unchanged on purpose: the era-lock violations keep their codes (`-32600` for a modern envelope on
a legacy-locked session, `-32022` for `initialize` on a modern-locked one), matching the Python SDK.

## How Has This Been Tested?

`test/mcp/request_envelope_test.rb` now covers: classification by the claim key alone,
a claimed-but-incomplete envelope staying modern, parsing without the optional `clientInfo`
(reader returns `nil`), `-32602` with the offending key names for a missing `clientCapabilities`
and for mistyped required and optional fields.

`test/mcp/server_test.rb` covers the dispatch-level behavior: a claimed
but incomplete envelope answers `-32602` naming the missing key, an envelope without `clientInfo` is
served with `server_context.client_info` reading `nil`, and a claim-less request on
a modern-locked session answers `-32602` naming the required keys.
The stdio and Streamable HTTP transport tests assert the new code on their envelope-requirement paths.

## Breaking Changes

None for conforming clients. Requests that were already rejected change error code
(`-32600` to `-32602`, HTTP status 400 unchanged), and requests carrying the `protocolVersion` claim key
with an incomplete envelope are now rejected as the spec mandates instead of being served as legacy requests.
Envelopes without `clientInfo`, previously rejected, now succeed.

test/mcp/server/transports/stdio_transport_test.rb

@@ -617,7 +617,7 @@ module MCP
           ])
 
           refute responses[0].key?(:error)
-          assert_equal JsonRpcHandler::ErrorCode::INVALID_REQUEST, responses[1].dig(:error, :code)
+          assert_equal JsonRpcHandler::ErrorCode::INVALID_PARAMS, responses[1].dig(:error, :code)
         end
 
         test "#send_request raises on a modern-locked session" do

test/mcp/server/transports/streamable_http_transport_test.rb

@@ -5425,7 +5425,7 @@ module MCP
           ))
 
           assert_equal 400, response[0]
-          assert_equal(-32600, JSON.parse(response[2][0]).dig("error", "code"))
+          assert_equal(-32602, JSON.parse(response[2][0]).dig("error", "code"))
         end
 
         test "modern POST maps a missing client capability to 400 with -32021" do