Fix EqualExportedValues on recursive values

[cyphercodes]

Summary

Fix EqualExportedValues so copying exported fields terminates for recursive values.

Changes

• Track visited pointer/slice/map values while copying exported fields.
• Reuse in-progress copies when a recursive value is encountered.
• Add a regression test for self-referential struct pointers with differing unexported fields.

Motivation

copyExportedFields recursed indefinitely on pointer cycles, causing EqualExportedValues to time out or overflow the stack for recursive data structures.

Exact test commands run:

• go test ./assert -run TestEqualExportedValuesRecursiveStruct -count=1 -timeout=3s (failed before the fix; timed out reproducing the bug)
• go test ./assert -run 'TestEqualExportedValuesRecursiveStruct|TestEqualExportedValues|TestObjectsExportedFieldsAreEqual|TestCopyExportedFields' -count=1
• go test ./assert -count=1
• go test ./... -count=1
• git diff --check

Related issues

Fixes #1915