Reject modern-removed methods before the connection era locks

## Motivation and Context

SEP-2575 removes `initialize`, `ping`, `logging/setLevel`, `resources/subscribe`,
and `resources/unsubscribe` from the modern lifecycle, and the server answers them with `-32601` there.
The check consulted `ServerSession#era` alone, but `StdioTransport` locks the era in `lock_modern_era_on_success`,
which runs after a response is produced. The first frame of a stdio connection therefore reached
the server with `era == nil`, and a request carrying the modern `_meta` envelope was served instead of refused.
All five methods leaked, one request per connection:

```
initialize             a full InitializeResult
ping                   {resultType: "complete"}
logging/setLevel       {resultType: "complete"}
resources/subscribe    {resultType: "complete"}
resources/unsubscribe  {resultType: "complete"}
```

`initialize` is the worst of the five because it also mutates the session: serving it negotiated
the legacy lifecycle for a request that had declared the modern one, pinning the connection to
`:legacy` and marking it initialized. The Streamable HTTP transport was never affected,
since it routes sessionless modern traffic to `handle_modern` and rejects a modern envelope on every other path.

The era is the wrong thing to consult on its own: a request declares which lifecycle it belongs to
through its own envelope, independently of connection state that settles later. The Python SDK gates
on exactly that, testing each request's `_meta` before dispatch rather than a negotiated connection
mode. `modern_request?` now does the same here, treating a locked `:modern` era as authoritative and
falling back to the request's envelope while the era is still undetermined. A legacy-locked session is left alone:
`lift_request_envelope` already answers a modern envelope there with the lifecycle violation,
which names the cause better than Method not found.

## How Has This Been Tested?

New tests in `test/mcp/server_test.rb` cover all five removed methods being refused with `-32601` when
they carry the envelope on an era-less session, an enveloped `initialize` no longer locking the era
or marking the session initialized, a method the modern lifecycle keeps still being served under the same
conditions, and a legacy `initialize` negotiating 2026-07-28 still succeeding and locking `:legacy` - negotiating
a dual-era version through the handshake is not the same as carrying the envelope. A test in
`test/mcp/server/transports/stdio_transport_test.rb` drives the case end to end,
sending an enveloped `resources/subscribe` as a connection's first frame.

One existing assertion changed. "modern results carry resultType complete" used `ping` as one of its
stamped examples, which only passed because of this defect; `ping` is absent from the 2026-07-28 schema,
so a modern request naming it is refused rather than stamped. It now uses `prompts/list`, with a note
on why `ping` cannot appear there. The existing legacy counterpart, which pings without an envelope,
is unchanged and still expects no stamp.

`bundle exec rake` (tests, RuboCop, and conformance baseline) passes. The conformance suite exercises
the HTTP transport, which this does not change, so its baseline is unaffected.

## Breaking Changes

None for a conforming client. A request that both carries the 2026-07-28 envelope and names a method
that revision removed now receives `-32601` instead of a result, which is the answer the spec already
required once the era was known.

lib/mcp/server.rb

@@ -565,9 +565,9 @@ module MCP
       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)
+      # there regardless of the server's declared capabilities: in this era the method does not exist
+      # at all, which is why the check precedes `ensure_capability!`.
+      if Methods::MODERN_REMOVED_METHODS.include?(method) && modern_request?(request, session)
         raise RequestHandlerError.new(
           "Method not found: #{method} is not part of the modern lifecycle (SEP-2575)",
           request,
@@ -677,6 +677,23 @@ module MCP
       }
     end
 
+    # Whether this request belongs to the modern lifecycle, used to refuse the RPCs SEP-2575 removed from it.
+    # A locked `ServerSession#era` is authoritative; until it locks, the request's own `_meta` envelope is
+    # the signal. Both are needed because `StdioTransport` locks the era only after a response succeeds,
+    # which would otherwise let the first request of a connection reach a method the modern lifecycle does not have.
+    # The Python SDK gates the same way, on the envelope of each request rather than on connection state that
+    # is only settled afterwards.
+    #
+    # A legacy-locked session is deliberately excluded: `lift_request_envelope` answers a modern envelope there
+    # with the lifecycle violation, which names the cause better than Method not found.
+    def modern_request?(request, session)
+      era = session.respond_to?(:era) ? session.era : nil
+      return true if era == :modern
+      return false unless era.nil?
+
+      RequestEnvelope.modern?(request.is_a?(Hash) ? request[:params] : nil)
+    end
+
     # Lifts the SEP-2575 per-request `_meta` envelope for modern requests. Only a request whose `_meta` carries
     # the full required triple is classified as modern; a partial triple keeps flowing through the legacy path untouched.
     # Notifications carry no envelope (their `_meta` is a `NotificationMetaObject`), and `server/discover` is