fix(cpp): address PR #858 review comments

- test_integration_vlm.cpp: replace GTEST_MESSAGE_(kNonFatalFailure)
  in expectPinnedLemonadeVersion() with std::cerr + SUCCEED() so missing
  version field or /system-info probe failure is purely informational and
  does not mark VLM tests as failed; real version mismatches still fail
  via EXPECT_EQ (blocking concern from review)
- types.cpp: expand detectImageMimeType short-buffer comment to explain
  why "image/png" fallback exists for null/<12 byte buffers (AC-15e
  contract, test fixtures) vs empty-string return for full-sized
  unrecognized magic (non-blocking concern from review)
- types.cpp: add BMP "BM" false-positive trade-off comment (minor nit)
- .gitattributes: mark cpp/tests/fixtures/*.png and *.jpg as binary to
  prevent LF/CRLF text-filter corruption (minor nit)

Author: itomek

.gitattributes

@@ -0,0 +1,3 @@
+# Binary test fixtures — prevent LF/CRLF mangling
+cpp/tests/fixtures/*.png binary
+cpp/tests/fixtures/*.jpg binary

cpp/src/types.cpp

@@ -13,8 +13,13 @@ namespace gaia {
 
 std::string detectImageMimeType(const std::uint8_t* data, std::size_t size) {
     // Buffers < 12 bytes cannot be safely probed for WebP (offset 8–11).
-    // Return "image/png" so callers treating these as padding/header stubs
-    // don't crash (AC-15e short-buffer safety contract).
+    // Returning "image/png" for null/short buffers is the AC-15e contract:
+    // test fixtures call this function directly with 1/5/11-byte header stubs
+    // and assert the safe fallback. In practice, Image::fromBytes already rejects
+    // empty byte vectors before calling this, and Image::fromFile reads the full
+    // file first — so neither factory can produce a mislabeled Image via this
+    // path. Full-sized buffers (>= 12 bytes) with unrecognized magic return ""
+    // so callers throw with a clear message.
     if (data == nullptr || size < 12) {
         return "image/png";
     }
@@ -38,6 +43,8 @@ std::string detectImageMimeType(const std::uint8_t* data, std::size_t size) {
         return "image/webp";
     }
     // BMP: "BM"
+    // Note: "BM" is a 2-byte signature with false-positive potential (any data starting "BM...").
+    // Acceptable here because Image::fromBytes enforces an explicit whitelist and callers supply real image files.
     if (data[0] == 'B' && data[1] == 'M') {
         return "image/bmp";
     }

cpp/tests/integration/test_integration_vlm.cpp

@@ -25,6 +25,7 @@
 #include <gaia/types.h>
 
 #include <cstdlib>
+#include <iostream>
 #include <string>
 
 #ifndef GAIA_TEST_FIXTURES_DIR
@@ -94,18 +95,22 @@ void expectPinnedLemonadeVersion() {
             v = info["lemonade_version"].get<std::string>();
         }
         if (v.empty()) {
-            GTEST_MESSAGE_("Lemonade /system-info did not return a version field; skipping pin check",
-                           ::testing::TestPartResult::kNonFatalFailure);
+            std::cerr << "[VLM version pin] Lemonade /system-info did not return a "
+                         "version field; skipping pin check (informational only)."
+                      << std::endl;
+            SUCCEED();
             return;
         }
         EXPECT_EQ(v, std::string(GAIA_PINNED_LEMONADE_VERSION))
             << "Lemonade server version (" << v << ") does not match "
             << "GAIA-pinned version (" << GAIA_PINNED_LEMONADE_VERSION << "). "
             << "Update LEMONADE_VERSION in src/gaia/version.py or roll back the server.";
     } catch (const std::exception& e) {
-        GTEST_MESSAGE_(("Lemonade /system-info probe failed: "
-                        + std::string(e.what())).c_str(),
-                       ::testing::TestPartResult::kNonFatalFailure);
+        std::cerr << "[VLM version pin] Lemonade /system-info probe failed: "
+                  << e.what()
+                  << " (informational only; continuing VLM test)."
+                  << std::endl;
+        SUCCEED();
     }
 }