feat(files): introduce upload_file tool for staging files and enhance save_file documentation

internal/tools/files_test.go

@@ -10,6 +10,7 @@ func TestDecodeBase64AcceptsWhitespaceAndMultipleVariants(t *testing.T) {
 	}{
 		{name: "standard with whitespace", input: "aG V s\nbG8=", want: "hello"},
 		{name: "raw standard", input: "aGVsbG8", want: "hello"},
+		{name: "standard with extra padding", input: "aGVsbG8==", want: "hello"},
 		{name: "standard url-safe payload", input: "--8=", want: string([]byte{0xfb, 0xef})},
 		{name: "raw url-safe payload", input: "--8", want: string([]byte{0xfb, 0xef})},
 	}