Enforce the modern lifecycle admission rules per SEP-2575

## Motivation and Context

The 2026-07-28 conformance requirements reject two admission gaps in the modern (stateless) lifecycle
that the current implementation answers with the wrong signal:

- The RPCs the modern lifecycle removes (`initialize`, `ping`, `logging/setLevel`, `resources/subscribe`, `resources/unsubscribe`)
  were still dispatched, or answered incidental errors (`initialize` answered `-32022`, `resources/subscribe` a capability error);
  the requirement is `-32601` Method not found with HTTP 404.
- `server/discover` was fully envelope-exempt, while the requirements validate its envelope
  like every other modern request.

The per-request `_meta` validation itself (`-32602` naming the offending keys, optional `clientInfo`,
era classification on the reserved `protocolVersion` key alone) landed separately in #491;
this change builds on that shape and completes the admission rules:

- `Server#handle_request` rejects `Methods::MODERN_REMOVED_METHODS` on a modern-era session
  with `-32601` before the capability check, since the method does not exist in that era at all.
  The Streamable HTTP modern path maps `-32601` to 404 and `-32602` to 400 through its existing status ladder.
- The dual-era sniff treats an `initialize` carrying the modern envelope as modern traffic (a modern client naming a removed method)
  instead of the legacy-distinctive handshake, and `server/discover` keeps its envelope exemption outside the modern era only.
  The envelope-less rejection on a modern-locked session adopts #491's wire shape, `-32602` with the detailed message.

One deliberate divergence from the Python SDK: its dual-era stream loop answers an `initialize` on
a modern-locked connection with `-32022` carrying the supported-version list, while its HTTP path serves
the 404/-32601 shape. The conformance requirement phrases removed methods transport-independently,
so this SDK answers `-32601` on every transport, stdio included, for uniform era semantics.

The stdio era-lock tests used a modern `ping` as the era-distinctive request; they now use `tools/list`,
since `ping` no longer exists in the modern lifecycle.

## How Has This Been Tested?

`bundle exec rake test` passes with zero failures and RuboCop reports no offenses.
New and updated tests cover each admission rule on `Server#handle`, the Streamable HTTP modern path, and the stdio era lock
(all five removed methods per transport, the envelope requirement on `server/discover`, and the envelope-carrying `initialize` routing).

Against the conformance fixture server, every admission check of the `server-stateless` scenario passes at
`--spec-version 2026-07-28`: the per-request `_meta` validation, removed-method 404, discovery, capability,
and version-negotiation checks are all green, and the only remaining failures are the `subscriptions/listen`
checks that belong to separate work. The `--requirements 2025-11-25` server leg passes 78/78, unchanged.

## Breaking Changes

None for stable protocol versions: legacy requests are byte-for-byte unchanged, and a bare legacy `_meta`
(`progressToken`, trace context) keeps its legacy classification. Within the modern lifecycle,
previously incidental error shapes are replaced by the ones the 2026-07-28 conformance requirements mandate.

lib/mcp/server.rb

@@ -561,6 +561,18 @@ module MCP
         return
       end
 
+      # SEP-2575 removes these RPCs from the modern lifecycle, so they answer with Method not found
+      # there regardless of the envelope or the server's declared capabilities: in this era the method
+      # does not exist at all, which is why the check precedes `ensure_capability!`.
+      if session.respond_to?(:era) && session.era == :modern && Methods::MODERN_REMOVED_METHODS.include?(method)
+        raise RequestHandlerError.new(
+          "Method not found: #{method} is not part of the modern lifecycle (SEP-2575)",
+          request,
+          error_type: :method_not_found,
+          error_code: JsonRpcHandler::ErrorCode::METHOD_NOT_FOUND,
+        )
+      end
+
       begin
         Methods.ensure_capability!(method, capabilities)
       rescue Methods::MissingRequiredCapabilityError => e
@@ -669,14 +681,13 @@ module MCP
     # an invalid request because a connection can never change eras.
     def lift_request_envelope(params, method:, session:)
       return if Methods.notification?(method)
-      return if method == Methods::SERVER_DISCOVER
 
       era = session.respond_to?(:era) ? session.era : nil
 
-      if era == :modern && method == Methods::INITIALIZE
-        requested = params.is_a?(Hash) ? params[:protocolVersion] || params["protocolVersion"] : nil
-        raise UnsupportedProtocolVersionError.new(requested, params)
-      end
+      # Outside the modern era, `server/discover` stays envelope-exempt so legacy connections can probe capabilities
+      # before any negotiation. Under the modern era every request carries the envelope, discovery included:
+      # the conformance suite's 2026-07-28 requirements reject an envelope-less `server/discover` with `-32602` (SEP-2575).
+      return if method == Methods::SERVER_DISCOVER && era != :modern
 
       if RequestEnvelope.modern?(params)
         if era == :legacy