fix(time-zone): a failed TryFind handed back the caller's previous zone (#2177, SR-AUD-224)

TryFindSystemTimeZoneById caught the lookup failure and returned false without
touching `result`, so a caller reusing one variable across several lookups was
handed the zone from an earlier, unrelated call. Measured before the fix:
TryFind("Mars/Olympus", out) returned false with out still holding "UTC".
Real .NET assigns null to the out parameter; the catch-all now resets it.

+6 regressions in TimeZoneInfoTests.cpp covering a populated out parameter, an
empty id, a path-traversal id, two consecutive failures, and both directions of
the success path. Mutation proven: deleting the reset fails exactly the four
pins that depend on it and leaves the two controls green.

Suite 114 -> 120, all passing. One inline header body; no public signature,
virtual, vtable, object layout or mangled symbol changed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014joExjgZv4aWFpzADeQGZm

Author: claude

modules/time-zone/include/System/TimeZoneInfo.hpp

@@ -564,13 +564,22 @@ namespace System {
          * @brief Tries to find a time zone by ID; returns false instead of throwing.
          *
          * C++ counterpart of .NET TimeZoneInfo.TryFindSystemTimeZoneById(string, out TimeZoneInfo).
+         *
+         * On failure @p result is set to @c nullptr, mirroring .NET's assignment of @c null to the
+         * @c out parameter. A caller that reuses one variable across several lookups therefore
+         * cannot be handed the previous zone by a lookup that failed.
+         *
+         * @param id     The time zone identifier to look up.
+         * @param result Receives the zone on success and @c nullptr on failure.
+         * @return true if the zone was found.
          */
         static bool TryFindSystemTimeZoneById(const std::string& id,
                                               std::shared_ptr<TimeZoneInfo>& result) {
             try {
                 result = FindSystemTimeZoneById(id);
                 return true;
             } catch (...) {
+                result.reset();
                 return false;
             }
         }

modules/time-zone/tests/System/TimeZoneInfoTests.cpp

@@ -606,3 +606,60 @@ TEST(TimeZoneInfoTests, TransitionTime_GetHashCode_FloatingRule) {
         tod, 10, 4, System::DayOfWeek::Sunday);
     EXPECT_EQ(t.GetHashCode(), 10 ^ (4 << 8));
 }
+
+// ---------------------------------------------------------------------------
+// Ticket #2177 (SR-AUD-224): a failed TryFindSystemTimeZoneById must clear the
+// caller's out parameter. Before this fix the catch-all returned false without
+// touching `result`, so a caller reusing one variable across several lookups was
+// handed the previous zone by a lookup that had failed. Measured before the fix
+// (build-probe/2176_probe1_surface.log): TryFind("Mars/Olympus", out) returned
+// false with out still holding "UTC". Real .NET assigns null to the out parameter.
+// ---------------------------------------------------------------------------
+
+TEST(TimeZoneInfoTests, TryFind_Failure_ClearsPreviouslyPopulatedOutParameter) {
+    std::shared_ptr<TimeZoneInfo> tz = TimeZoneInfo::FindSystemTimeZoneById("UTC");
+    ASSERT_NE(tz, nullptr);
+    ASSERT_EQ(tz->getIdProperty(), "UTC");
+    EXPECT_FALSE(TimeZoneInfo::TryFindSystemTimeZoneById("Mars/Olympus", tz));
+    EXPECT_EQ(tz, nullptr);
+}
+
+TEST(TimeZoneInfoTests, TryFind_Failure_EmptyId_ClearsOutParameter) {
+    std::shared_ptr<TimeZoneInfo> tz = TimeZoneInfo::FindSystemTimeZoneById("UTC");
+    ASSERT_NE(tz, nullptr);
+    EXPECT_FALSE(TimeZoneInfo::TryFindSystemTimeZoneById("", tz));
+    EXPECT_EQ(tz, nullptr);
+}
+
+TEST(TimeZoneInfoTests, TryFind_Failure_PathTraversal_ClearsOutParameter) {
+    std::shared_ptr<TimeZoneInfo> tz = TimeZoneInfo::FindSystemTimeZoneById("UTC");
+    ASSERT_NE(tz, nullptr);
+    EXPECT_FALSE(TimeZoneInfo::TryFindSystemTimeZoneById("../../etc/passwd", tz));
+    EXPECT_EQ(tz, nullptr);
+}
+
+TEST(TimeZoneInfoTests, TryFind_Failure_AfterFailureLeavesNullNotStale) {
+    // Two failures in a row must both leave the parameter null, not resurrect anything.
+    std::shared_ptr<TimeZoneInfo> tz;
+    EXPECT_FALSE(TimeZoneInfo::TryFindSystemTimeZoneById("Mars/Olympus", tz));
+    EXPECT_EQ(tz, nullptr);
+    EXPECT_FALSE(TimeZoneInfo::TryFindSystemTimeZoneById("Venus/Maxwell", tz));
+    EXPECT_EQ(tz, nullptr);
+}
+
+TEST(TimeZoneInfoTests, TryFind_Success_OverwritesAPreviousZone) {
+    // The success path must still replace whatever was there.
+    std::shared_ptr<TimeZoneInfo> tz = TimeZoneInfo::FindSystemTimeZoneById("Europe/Prague");
+    ASSERT_NE(tz, nullptr);
+    ASSERT_TRUE(TimeZoneInfo::TryFindSystemTimeZoneById("UTC", tz));
+    ASSERT_NE(tz, nullptr);
+    EXPECT_EQ(tz->getIdProperty(), "UTC");
+}
+
+TEST(TimeZoneInfoTests, TryFind_SuccessThenFailure_DoesNotKeepTheSuccess) {
+    std::shared_ptr<TimeZoneInfo> tz;
+    ASSERT_TRUE(TimeZoneInfo::TryFindSystemTimeZoneById("Europe/Prague", tz));
+    ASSERT_NE(tz, nullptr);
+    EXPECT_FALSE(TimeZoneInfo::TryFindSystemTimeZoneById("Europe/Nowhere", tz));
+    EXPECT_EQ(tz, nullptr);
+}