[arthurscchan]

This PR creates a new OSS-Fuzz fuzzer with corpus/dict targets xml processing, together with google/oss-fuzz#16436 to enable a new fuzzer for proejct BehaviorTree.CPP.

[facontidavide]

Thanks @arthurscchan. As it stands this target can't find the bugs it's aimed at:

1. There's no round-trip check. Modes 1–2 export, re-import and export again, but never compare the results. A logic divergence (a lost name, a dropped port) goes unnoticed. For example #1262 (SubTree name dropped by WriteTreeToXML, fixed in #1268) would not have been found.
2. A failed re-import is swallowed. If factory2.registerBehaviorTreeFromText(exported) or createTree throws, that means BT.CPP can't read its own output, which is a real bug. The outer catch(const std::exception&) hides it. Exceptions are fine for the generated input, but not for the re-import.
3. Mode 0 duplicates bt_fuzzer (raw bytes into the parser).
4. writeTreeXSD(factory) and writeTreeNodesModelXML act on a fixed factory, so the result doesn't depend on the input. It only slows each run.

Suggested shape: generate a valid tree, exported1 = WriteTreeToXML(tree, false, false), re-import it in a fresh factory outside the try/catch (or abort() in its catch), exported2 = WriteTreeToXML(tree2, false, false), then if(exported1 != exported2) abort();. Drop mode 0 and the XSD/model writers. Happy to merge that version.