Bound OAuth response bodies in the client

## Motivation and Context

`MCP::Client::HTTP` caps a single message from the server at 4 MiB, but the OAuth flow talks to
authorization endpoints over its own Faraday connection, and that one had no cap. Discovery,
dynamic client registration, and token responses were buffered in full and handed to `JSON.parse`,
so a server that answered an OAuth request with an endless body grew the client's memory for as
long as it kept sending.

`Flow` and `IDJAGTokenExchange` each build their own connection, so both needed the bound.
The new `BoundedBody` counts bytes in a Faraday `on_data` callback and refuses the response once
it passes `MAX_RESPONSE_BYTES`, which matches the 4 MiB of `MCP::Client::HTTP::MAX_MESSAGE_BYTES`
and `MCP::Client::Stdio::MAX_LINE_BYTES`. It hands back the status paired with the bounded body
rather than the Faraday response, so no later caller can reach the unbounded `response.body`.

Those bytes are counted after decompression. The default Net::HTTP adapter negotiates `Accept-Encoding`
itself  and reads the body through `Net::HTTPResponse#inflater`, so a small compressed body that expands past
the cap is refused partway through the expansion. That holds only while the connection leaves
`Accept-Encoding` alone, which is now recorded in a comment on both `default_http_client` definitions.

An over-cap response is refused rather than truncated: a partial discovery or token document cannot be validated.
Protected resource metadata is the one place where refusing is not fatal, since any PRM discovery failure
already selects the legacy authorization path by design.

## How Has This Been Tested?

New unit tests in `test/mcp/client/oauth/bounded_body_test.rb` cover the streaming path,
the at-limit boundary, and the fallback for adapters that ignore `on_data`. One of them serves
a gzip body over a real socket rather than through WebMock, which returns a stubbed body verbatim
and never runs the inflater; it fails if the cap ever starts measuring compressed bytes. New tests in
`test/mcp/client/oauth/flow_test.rb` and `test/mcp/client/oauth/id_jag_token_exchange_test.rb` drive each of
the four request wrappers end to end with an over-cap body; all five fail without the cap in place.

## Breaking Changes

An OAuth endpoint response larger than 4 MiB is now rejected instead of parsed.

lib/mcp/client/oauth/flow.rb

@@ -1021,26 +1021,54 @@ module MCP
         end
 
         def http_get(url)
-          http_client.get(url)
+          bounded_request do |on_data|
+            http_client.get(url) do |req|
+              req.options.on_data = on_data
+            end
+          end
         end
 
         def http_post_json(url, body)
-          http_client.post(url) do |req|
-            req.headers["Content-Type"] = "application/json"
-            req.headers["Accept"] = "application/json"
-            req.body = JSON.generate(body)
+          bounded_request do |on_data|
+            http_client.post(url) do |req|
+              req.headers["Content-Type"] = "application/json"
+              req.headers["Accept"] = "application/json"
+              req.options.on_data = on_data
+              req.body = JSON.generate(body)
+            end
           end
         end
 
         def http_post_form(url, form, headers: {})
-          http_client.post(url) do |req|
-            req.headers["Content-Type"] = "application/x-www-form-urlencoded"
-            req.headers["Accept"] = "application/json"
-            headers.each { |key, value| req.headers[key] = value }
-            req.body = URI.encode_www_form(form)
+          bounded_request do |on_data|
+            http_client.post(url) do |req|
+              req.headers["Content-Type"] = "application/x-www-form-urlencoded"
+              req.headers["Accept"] = "application/json"
+
+              headers.each do |key, value|
+                req.headers[key] = value
+              end
+
+              req.options.on_data = on_data
+              req.body = URI.encode_www_form(form)
+            end
           end
         end
 
+        # Issues a request with the response body bounded as it arrives, and returns the status paired with
+        # that body. An over-cap response is refused rather than truncated: a partial discovery or token document
+        # cannot be validated, and `fetch_metadata_json` must not fall through to the next candidate URL either,
+        # since the same server would serve the same body.
+        def bounded_request
+          bounded = BoundedBody.new
+
+          response = yield(bounded.on_data)
+
+          bounded.response_for(response)
+        rescue BoundedBody::TooLargeError => e
+          raise AuthorizationError, "#{e.message}."
+        end
+
         def http_client
           @http_client ||= @http_client_factory.call
         end
@@ -1050,8 +1078,14 @@ module MCP
         # that transparently followed a `3xx` would let a server reach a host the checks just refused.
         # A caller passing `http_client_factory:` takes on that responsibility: add redirect following here
         # and the guards above only cover the first hop.
+        #
+        # `Accept-Encoding` is deliberately left unset. `Net::HTTP::GenericRequest` negotiates it and decodes
+        # the response only while the caller has not claimed that header; assigning it turns `decode_content` off,
+        # which would silently move `BoundedBody`'s cap onto compressed bytes and let a small body expand past it
+        # after the check.
         def default_http_client
           require "faraday"
+
           Faraday.new do |faraday|
             faraday.headers["Accept"] = "application/json"
           end