Validate the RFC 9207 iss Authorization Response Parameter per SEP-2468

## Motivation and Context

SEP-2468 (modelcontextprotocol/modelcontextprotocol#2468, merged for the 2026-07-28 spec release) recommends
that authorization servers include the RFC 9207 `iss` parameter in authorization responses and requires MCP clients
to validate a present `iss` against the expected issuer before redeeming the authorization code.
This mitigates mix-up attacks in multi-IdP setups, where an attacker-controlled authorization server could otherwise capture
a code minted by an honest server.

This mirrors the TypeScript SDK's approach (typescript-sdk#2272, with typescript-sdk#1957 as the reference implementation),
adapted to the Ruby provider contract:

- `Provider`'s `callback_handler` may now return `[code, state, iss]` in addition to the existing `[code, state]`.
  The 3-element form opts into validation; returning `iss = nil` asserts the caller inspected the authorization response
  and found no `iss`.
- `Flow#run!` validates the issuer after the `state` check and before `exchange_authorization_code`,
  implementing the RFC 9207 Section 2.4 rules: a present `iss` must equal the AS metadata `issuer` exactly
  (the value `ensure_issuer_matches!` already pinned to the discovery URL); a nil `iss` from a 3-element callback is rejected
  when the AS metadata advertises `authorization_response_iss_parameter_supported: true`; a legacy 2-element callback skips the check,
  so every existing integration keeps working. On any violation the flow aborts with `AuthorizationError` before
  the code reaches a token endpoint.
- The conformance client now returns the 3-element form, plumbing `iss` through from the redirect URL.

Resolves #386.

## How Has This Been Tested?

New tests in `test/mcp/client/oauth/flow_test.rb`:

- a matching `iss` completes the flow and persists tokens
- a mismatched `iss` raises `AuthorizationError` and never contacts the token endpoint (`assert_not_requested` on the token POST)
- a nil `iss` with `authorization_response_iss_parameter_supported: true` raises without contacting the token endpoint (fail closed)
- a legacy 2-element callback completes even when the AS advertises `iss` support (backward compatibility)
- a nil `iss` without the advertisement completes
- a mismatched `iss` is rejected even without the advertisement (validate-when-present is unconditional)

`bundle exec rake` (tests, RuboCop, and conformance baseline with the 3-element conformance callback) passes.

## Breaking Changes

None. The 2-element `callback_handler` contract continues to work unchanged; issuer validation activates only
when a callback opts in by returning the 3-element form.

conformance/client.rb

@@ -95,7 +95,9 @@ def build_oauth_provider(context, scenario:)
 
   callback_handler = -> do
     query = URI.decode_www_form(callback_holder.fetch(:url).query).to_h
-    [query["code"], query["state"]]
+    # The 3-element form opts into SEP-2468 / RFC 9207 `iss` validation;
+    # `query["iss"]` is nil when the authorization response carried no `iss`.
+    [query["code"], query["state"], query["iss"]]
   end
 
   MCP::Client::OAuth::Provider.new(