Harden stl_emulation span assignment, make_span deduction, and bounds safety - #9349

Description

This change hardens the STL emulation utility header (include/flatbuffers/stl_emulation.h):

1. Span assignment & rebinding: In flatbuffers::span, data_ was declared pointer const data_, making copy assignment ill-formed when instantiated. Furthermore, operator= was missing a return statement (return *this;). Fixed data_ to be non-const so the span view is rebindable (matching C++20 std::span) and added return *this;.
2. make_span deduction: Removed the non-deducible Extent template parameter from the pointer overloads of make_span(first, count), allowing caller template argument deduction without requiring manual instantiation of <ElementType, Extent>.
3. Span bounds safety: Added FLATBUFFERS_ASSERT(idx < size()) in span::operator[] to catch out-of-bounds accesses.
4. Optional safety: Added FLATBUFFERS_ASSERT(has_value()) in Optional::operator*() to guard against dereferencing an empty optional.

Testing

• Added StlEmulationSafetyTest in tests/test.cpp verifying span copy-assignment rebinding, make_span template deduction on mutable and const pointers, and optional value access.
• All unit tests in flattests pass.