fix: report the full key path in MergeStrict type-mismatch error

[koriyoshi2041]

mergeStrict (maps/maps.go) builds newFullKey, the full dotted path to the key currently being merged, and passes it to the recursive call. But the type-mismatch error reports the parent fullKey instead:

return fmt.Errorf("incorrect types at key %v, type %T != %T", fullKey, b[key], val)

So the error names the wrong key:

• A top-level type conflict has fullKey == "", producing incorrect types at key , type int != string (blank key name).
• A nested conflict reports the enclosing map path and drops the actual conflicting leaf, e.g. parent.child instead of parent.child.key.

This is reachable from public API via MergeStrict and Koanf.Load with Conf{StrictMerge: true}. The fix uses newFullKey, matching what the recursive call already uses.

Added TestMergeStrictErrorKey in tests/maps_test.go asserting the error names the full path for both a top-level and a nested conflict (the existing TestMergeStrict only checked that an error occurred, not its contents). go test passes.