chat-parser: handle whitespace around JSON in tool call parsing

Models often output whitespace (newlines, spaces) between XML tags and
JSON content, e.g.:

    <function=transfer_to_human>
         {"reason": "..."}
    </function>

This was causing parsing failures in Hermes 2 Pro and other formats
because try_consume_json() expected JSON to start immediately at the
current position.

Fix by modifying try_consume_json() to:
- Skip leading whitespace before attempting to parse JSON
- Consume trailing whitespace after successful JSON parse
- Restore position if JSON parsing fails (important for streaming)

This fix benefits all chat formats that use try_consume_json().

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

common/chat-parser.cpp

@@ -488,13 +488,24 @@ std::optional<common_chat_msg_parser::find_regex_result> common_chat_msg_parser:
 }
 
 std::optional<common_json> common_chat_msg_parser::try_consume_json() {
+    // Skip leading whitespace - JSON values never start with whitespace, and models often
+    // output newlines/spaces between tags and JSON content (e.g., "<function=name>\n  {...}")
+    auto saved_pos = pos_;
+    consume_spaces();
+
     auto it = input_.cbegin() + pos_;
     const auto end = input_.cend();
     common_json result;
     if (!common_json_parse(it, end, healing_marker_, result)) {
+        // Restore position if parsing failed - don't consume whitespace without a successful parse
+        pos_ = saved_pos;
         return std::nullopt;
     }
     pos_ = std::distance(input_.cbegin(), it);
+
+    // Consume trailing whitespace after successful JSON parse
+    consume_spaces();
+
     if (result.healing_marker.marker.empty()) {
         // No healing marker, just return the parsed json
         return result;

tests/test-chat.cpp

@@ -1018,6 +1018,15 @@ static void test_template_output_parsers() {
                 "<function=special_function>{\"arg1\": 1}</function>",
                 /* is_partial= */ false,
                 {COMMON_CHAT_FORMAT_HERMES_2_PRO}));
+        // Test <function=name> with whitespace before JSON (issue: model outputs newline+spaces before JSON)
+        assert_msg_equals(
+            message_assist_call,
+            common_chat_parse(
+                "<function=special_function>\n"
+                "     {\"arg1\": 1}\n"
+                "</function>",
+                /* is_partial= */ false,
+                {COMMON_CHAT_FORMAT_HERMES_2_PRO}));
         assert_msg_equals(
             message_assist_call,
             common_chat_parse(