Let a provider customize the HTTP client the OAuth flow uses

## Motivation and Context

`MCP::Client::OAuth::Flow` takes an `http_client_factory:`, but `MCP::Client::HTTP` builds
the flow itself and passes none, so an application using `oauth:` could not add middleware to
the requests the flow makes: Protected Resource Metadata and authorization server metadata discovery,
dynamic client registration, and every token request.
The block that customizes the transport's Faraday connection does not help,
because that connection is bound to the MCP server URL and carries the `headers:` meant for that server,
and the authorization server is a different origin. The TypeScript SDK hands the transport's `fetch` to `auth()`
and the Python SDK sends the provider's requests through the transport's httpx client, so both let
an application instrument these requests; this SDK did not. (#558)

Providers now take `http_client_customizer:`, a callable invoked with the Faraday connection
the flow builds for its own requests, next to the other callables a provider already holds.
It runs after the SDK's defaults (`Accept: application/json`, no redirect following) and
between two middlewares of the SDK's own: `Flow::RequestedURLStamp`, registered before the customizer is invoked,
records the URL the flow asked for on the request environment, and `Flow::RequestedOriginGuard`, registered last,
sees what the customizer's middleware did to the request after that record. A request that would leave that origin,
by following a redirect or rewriting the URL, or that arrives without the record, is refused with
`Flow::DestinationMismatchError` before it reaches the adapter. The record lives on the environment rather than
in `env.request.context`, which stays the application's to fill or replace. The error is an `ArgumentError`
rather than an `AuthorizationError` so neither discovery nor the transport's refresh fallback can absorb
a configuration mistake. The boundary resembles the one the Python SDK keeps for its own requests,
which follows a redirect only within the origin; this flow follows none itself.
`Flow.build_http_client` assembles that connection and `Flow.new(http_client_factory:)` keeps replacing it whole.
Following done inside an adapter cannot be seen by any guard, so the documentation keeps that, `Accept-Encoding`,
and `raise_error` off the list of things a customizer may add, and notes that a recording middleware sees
client credentials and tokens.

Fixes #558.

## How Has This Been Tested?

New tests in `test/mcp/client/oauth/flow_test.rb`, `test/mcp/client/oauth/http_oauth_test.rb`,
and the three provider test files. Against the library before this change, every one of them fails,
on the unknown keyword or on the missing reader.

## Breaking Changes

None. The customizer is opt-in, and the guard cannot fire on the SDK's own middleware-free connection.

lib/mcp/client/oauth/provider.rb

@@ -53,6 +53,8 @@ module MCP
       #   the grant itself. The authorization request is not affected. A key in `Flow::RESERVED_TOKEN_REQUEST_PARAMS`
       #   raises `Flow::InvalidTokenRequestParamsError`.
       #   The Hash is copied and frozen. See `StorageBackedProvider#token_request_params`.
+      # - `http_client_customizer` - Callable invoked with the `Faraday::Connection` the flow builds for
+      #   its own requests; see `StorageBackedProvider#http_client_customizer`.
       class Provider
         include StorageBackedProvider
 
@@ -93,7 +95,8 @@ module MCP
           storage: nil,
           client_id_metadata_document_url: nil,
           authorization_request_validator: nil,
-          token_request_params: nil
+          token_request_params: nil,
+          http_client_customizer: nil
         )
           unless Discovery.secure_url?(redirect_uri)
             raise InsecureRedirectURIError,
@@ -115,6 +118,8 @@ module MCP
                 "per the MCP authorization specification and `draft-ietf-oauth-client-id-metadata-document`."
           end
 
+          http_client_customizer = validated_http_client_customizer(http_client_customizer)
+
           @client_metadata = client_metadata
           @redirect_uri = redirect_uri
           @redirect_handler = redirect_handler
@@ -124,6 +129,7 @@ module MCP
           @client_id_metadata_document_url = client_id_metadata_document_url
           @authorization_request_validator = authorization_request_validator
           @token_request_params = frozen_token_request_params(token_request_params)
+          @http_client_customizer = http_client_customizer
         end
 
         # Identifies the OAuth flow this provider drives.