Fix RuboCop offenses detected with rubocop-shopify 3.0.0

## Motivation and Context

This repository does not track Gemfile.lock, so `bundle install` resolves to the latest RuboCop gems.
Upgrading rubocop-shopify from 2.x to 3.0.0 surfaces 8 new offenses because the shared config no longer
disables `Style/MutableConstant`, newly disables `Lint/RescueException` and `Naming/AccessorMethodName`,
and drops `AllowedPatterns` for `Naming/MethodName`. Upgrading rubocop itself to 1.88 surfaces one more offense
from the new `Style/ReduceToHash` cop.

This change fixes them as follows:

- Freeze mutable constants. Regexp constants are also flagged because `TargetRubyVersion` is 2.7,
  where Regexp literals are not frozen.
- Remove inline disable directives that became redundant.
- Restore `AllowedPatterns: ['\Atest_']` for `Naming/MethodName` so `def test_` style test names
  can reference class names such as `NoMethodError`.
- Swap out the frozen `SUPPORTED_STABLE_PROTOCOL_VERSIONS` constant in the protocol version fallback test
  since Mocha cannot stub methods on frozen objects.
- Use `to_h` instead of `each_with_object` in `Server#index_resources_by_uri`.

Ref: https://rubygems.org/gems/rubocop-shopify/versions/3.0.0

## How Has This Been Tested?

`bundle exec rake` passes: 1402 runs, 0 failures, 0 errors, and RuboCop reports no offenses.
The conformance suite also passes against its baseline.

## Breaking Changes

None. Freezing these constants only affects code that mutates them in place, which has never been supported.

test/mcp/server/transports/stdio_notification_integration_test.rb

@@ -27,7 +27,7 @@ module MCP
             nil # Simulate end of input
           end
 
-          def set_encoding(encoding) # rubocop:disable Naming/AccessorMethodName
+          def set_encoding(encoding)
             # Mock implementation
           end
 

test/mcp/server/transports/streamable_http_transport_test.rb

@@ -1925,7 +1925,10 @@ module MCP
         end
 
         test "missing MCP-Protocol-Version header falls back to default for validation" do
-          MCP::Configuration::SUPPORTED_STABLE_PROTOCOL_VERSIONS.stubs(:include?).returns(false)
+          # The constant is frozen, so swap it out instead of stubbing `include?` on it.
+          original_versions = MCP::Configuration::SUPPORTED_STABLE_PROTOCOL_VERSIONS
+          MCP::Configuration.send(:remove_const, :SUPPORTED_STABLE_PROTOCOL_VERSIONS)
+          MCP::Configuration.const_set(:SUPPORTED_STABLE_PROTOCOL_VERSIONS, [].freeze)
 
           request = Rack::Request.new(
             "REQUEST_METHOD" => "POST",
@@ -1938,6 +1941,9 @@ module MCP
 
           body = JSON.parse(response[2][0])
           assert_includes body["error"]["message"], MCP::Configuration::DEFAULT_NEGOTIATED_PROTOCOL_VERSION
+        ensure
+          MCP::Configuration.send(:remove_const, :SUPPORTED_STABLE_PROTOCOL_VERSIONS)
+          MCP::Configuration.const_set(:SUPPORTED_STABLE_PROTOCOL_VERSIONS, original_versions)
         end
 
         test "POST request with empty MCP-Protocol-Version header returns 400" do