Improve PKCE from_h docs and string-key test

lib/mcp_client/auth.rb

@@ -308,9 +308,12 @@ def to_h
       end
 
       # Create PKCE instance from hash
-      # @param data [Hash] Hash with PKCE parameters
+      # @param data [Hash] Hash with PKCE parameters (symbol or string keys)
       # @return [PKCE] New PKCE instance
       # @raise [ArgumentError] If required parameters are missing
+      # @note code_challenge_method is optional and defaults to 'S256'.
+      #   The code_challenge is not re-validated against code_verifier;
+      #   callers are expected to provide values from a prior to_h round-trip.
       def self.from_h(data)
         verifier = data[:code_verifier] || data['code_verifier']
         challenge = data[:code_challenge] || data['code_challenge']

spec/lib/mcp_client/auth_spec.rb

@@ -138,14 +138,23 @@
     end
 
     describe '#to_h and .from_h' do
-      it 'round-trips data with symbol and string keys' do
+      it 'round-trips data with symbol keys' do
         original = described_class.new
         restored = described_class.from_h(original.to_h)
         expect(restored.code_verifier).to eq(original.code_verifier)
         expect(restored.code_challenge).to eq(original.code_challenge)
         expect(restored.code_challenge_method).to eq('S256')
       end
 
+      it 'round-trips data with string keys' do
+        original = described_class.new
+        string_hash = original.to_h.transform_keys(&:to_s)
+        restored = described_class.from_h(string_hash)
+        expect(restored.code_verifier).to eq(original.code_verifier)
+        expect(restored.code_challenge).to eq(original.code_challenge)
+        expect(restored.code_challenge_method).to eq('S256')
+      end
+
       it 'raises error when required fields are missing' do
         expect { described_class.from_h({}) }.to raise_error(ArgumentError)
       end