fix: detect pointer cycles in copyExportedFields to prevent stack overflow

[tamish560]

What

copyExportedFields now tracks visited pointer addresses to detect and break pointer cycles, preventing stack overflow on recursive data structures.

Why

EqualExportedValues recurses through a value's exported fields via copyExportedFields. When the value contains a pointer cycle (a struct that directly or indirectly points back to itself), the function follows the cycle indefinitely and exhausts the goroutine stack, killing the entire test binary.

Closes #1915

How

1. Extracted the recursion into copyExportedFieldsWithVisited which accepts a map[uintptr]bool of visited pointer addresses.
2. For pointer-typed values, the pointer address is checked against the visited map. If already seen, reflect.Zero of the pointer type is returned, breaking the cycle.
3. copyExportedFields calls copyExportedFieldsWithVisited with a fresh map.
4. All existing call sites remain unchanged.

The returned zero value for cyclic pointers means both a and b in a cycle like a.Self = a / b.Self = b get Self = nil in their copies, so the comparison reports them as equal.

Testing

Added TestEqualExportedValuesRecursiveStruct which constructs a self-referential struct and verifies EqualExportedValues completes without stack overflow. All existing tests pass unchanged.

[tamish560]

Closing this as a duplicate of #1916 by @cyphercodes, which was opened 19 days earlier and addresses the same issue (#1915). Should have checked for existing PRs before opening this one.