Bound stdio frame reads with max_line_bytes

## Motivation and Context

CRuby's `IO#gets` without a limit reads until the next newline with no upper bound.
A peer that never emits a newline can therefore grow a single String until the process
is killed by the OS OOM killer.

This is a denial-of-service exposure (GHSA-7683-3w9x-ch42), identified during
a cross-SDK audit of the same unbounded-buffer pattern across the MCP SDKs.
It is most relevant when a host application spawns a sandboxed or untrusted MCP server
and pipes its stdout into an unconstrained host process, where a memory-capped peer
can exhaust the host.

Each frame is now capped at a configurable `max_line_bytes` (default 4 MiB,
large enough for any realistic frame including base64-embedded images,
and the value recommended in the advisory). A frame that reaches the limit without
a newline is treated as a transport error: the client raises, and the server reports
the error and closes the connection. A short final frame that ends at EOF without
a trailing newline is still accepted, so legitimate traffic is unaffected.

Both constructors validate `max_line_bytes:` as a positive Integer,
since `nil` or a non-positive value makes `IO#gets` read without an upper bound and
would silently disable the protection.

## How Has This Been Tested?

Server tests cover the `open` read loop stopping and reporting on an over-limit frame,
the `send_request` read loop raising on an over-limit response, and a valid final frame
without a trailing newline still being processed.

Client tests cover `read_response` raising on an over-limit response frame and
a normal response still round-tripping within a custom `max_line_bytes`.

Both transports also have tests rejecting invalid `max_line_bytes:` values
(`nil`, zero, negative, non-Integer) with an `ArgumentError`.

The full test suite and RuboCop pass.

## Breaking Changes

None. The default 4 MiB limit is far larger than any realistic JSON-RPC frame,
so existing well-behaved traffic is unaffected. The new `max_line_bytes:` keyword
on both transport constructors is purely additive and optional.

test/mcp/server/transports/stdio_transport_test.rb

@@ -441,6 +441,91 @@ module MCP
             end
           end
         end
+
+        test "raises ArgumentError when max_line_bytes is not a positive Integer" do
+          [nil, 0, -1, 1.5, "1024"].each do |invalid|
+            error = assert_raises(ArgumentError) do
+              StdioTransport.new(@server, max_line_bytes: invalid)
+            end
+            assert_equal("max_line_bytes must be a positive Integer", error.message)
+          end
+        end
+
+        test "open stops and reports when a stdin frame exceeds max_line_bytes without a newline" do
+          transport = StdioTransport.new(@server, max_line_bytes: 64)
+          input = StringIO.new("A" * 200) # No newline: an unbounded frame.
+          output = StringIO.new
+
+          reported = []
+          original_stdin = $stdin
+          original_stdout = $stdout
+          original_reporter = MCP.configuration.exception_reporter
+
+          begin
+            $stdin = input
+            $stdout = output
+            MCP.configuration.exception_reporter = ->(e, ctx) { reported << [e, ctx] }
+
+            transport.open
+
+            refute(transport.instance_variable_get(:@open))
+            assert_equal(1, reported.size)
+            error, _ctx = reported.first
+            assert_instance_of(RequestHandlerError, error)
+            assert_match(/exceeds 64 bytes without a newline/, error.message)
+          ensure
+            $stdin = original_stdin
+            $stdout = original_stdout
+            MCP.configuration.exception_reporter = original_reporter
+          end
+        end
+
+        test "open processes a valid final frame that ends at EOF without a trailing newline" do
+          request = { jsonrpc: "2.0", method: "ping", id: "eof" }
+          input = StringIO.new(JSON.generate(request)) # No trailing newline.
+          output = StringIO.new
+
+          original_stdin = $stdin
+          original_stdout = $stdout
+
+          begin
+            $stdin = input
+            $stdout = output
+
+            @transport.open
+
+            response = JSON.parse(output.string, symbolize_names: true)
+            assert_equal("eof", response[:id])
+            assert_equal({}, response[:result])
+          ensure
+            $stdin = original_stdin
+            $stdout = original_stdout
+          end
+        end
+
+        test "send_request raises when a response frame exceeds max_line_bytes without a newline" do
+          transport = StdioTransport.new(@server, max_line_bytes: 64)
+          input = StringIO.new("A" * 200) # No newline: an unbounded frame.
+          output = StringIO.new
+
+          original_stdin = $stdin
+          original_stdout = $stdout
+
+          begin
+            $stdin = input
+            $stdout = output
+            transport.instance_variable_set(:@open, true)
+
+            error = assert_raises(RequestHandlerError) do
+              transport.send_request("sampling/createMessage", { messages: [] })
+            end
+
+            assert_match(/exceeds 64 bytes without a newline/, error.message)
+          ensure
+            $stdin = original_stdin
+            $stdout = original_stdout
+          end
+        end
       end
     end
   end