spotbugs(RCN+REC) ChatRequest source cleanup (4 cleared)

NullAway and the package's @NullMarked declaration prove the three
getters (ChatMessage.getContent, ToolCall.getArgumentsJson,
ToolDefinition.getDescription) are @nonnull at compile time, so the
runtime "value == null ? "" : value" ternaries and the
"if (getDescription() != null)" guard at three sites in
buildMessagesJson / buildToolsJson are dead.

Also narrows the catch in buildToolsJson from Exception to IOException.
The only checked exception MAPPER.readTree(String) can throw is
JsonProcessingException, which extends IOException — narrowing is
honest and removes the catch-Exception code smell.

Adds: import java.io.IOException.

Compatibility:
- Empty-string output on a null content/arguments is impossible per
  NullAway, so removing the fallback never changes runtime behaviour.
- The 7 tests in ChatResponseTest (including
  buildMessagesJsonRoundTripsToolTurns and
  buildToolsJsonInlinesParameterSchema, which exercise both modified
  code paths) stay green.

SpotBugs Max+Low:
  - RCN_REDUNDANT_NULLCHECK_OF_NONNULL_VALUE: 4 -> 1
    (ChatMessage.requireNonEmpty:103 remains, separate cluster)
  - REC_CATCH_EXCEPTION: 1 -> 0
Total jllama: 19 -> 15.

Author: claude

src/main/java/net/ladenthin/llama/ChatRequest.java

@@ -7,6 +7,7 @@
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.fasterxml.jackson.databind.node.ArrayNode;
 import com.fasterxml.jackson.databind.node.ObjectNode;
+import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
@@ -167,7 +168,7 @@ public String buildMessagesJson() {
         for (ChatMessage m : messages) {
             ObjectNode obj = MAPPER.createObjectNode();
             obj.put("role", m.getRole());
-            obj.put("content", m.getContent() == null ? "" : m.getContent());
+            obj.put("content", m.getContent());
             final String toolCallId = m.getToolCallId();
             if (toolCallId != null) {
                 obj.put("tool_call_id", toolCallId);
@@ -180,7 +181,7 @@ public String buildMessagesJson() {
                     entry.put("type", "function");
                     ObjectNode fn = MAPPER.createObjectNode();
                     fn.put("name", call.getName());
-                    fn.put("arguments", call.getArgumentsJson() == null ? "" : call.getArgumentsJson());
+                    fn.put("arguments", call.getArgumentsJson());
                     entry.set("function", fn);
                     tc.add(entry);
                 }
@@ -204,10 +205,10 @@ public Optional<String> buildToolsJson() {
             entry.put("type", "function");
             ObjectNode fn = MAPPER.createObjectNode();
             fn.put("name", t.getName());
-            if (t.getDescription() != null) fn.put("description", t.getDescription());
+            fn.put("description", t.getDescription());
             try {
                 fn.set("parameters", MAPPER.readTree(t.getParametersSchemaJson()));
-            } catch (Exception e) {
+            } catch (IOException e) {
                 fn.put("parameters", t.getParametersSchemaJson());
             }
             entry.set("function", fn);