Fix incorrect const qualification for tbm_add_tuples() and itemptr_to_uint64()

On 29.10.25 04:11, Chao Li wrote:

I noticed a wrong const qualification:
```
void
tbm_add_tuples(TIDBitmap *tbm, const ItemPointer tids, int ntids,
  bool recheck)
```

This "const" only protects "tids" itself from updating, which is
meaningless. I believe the real intention should be protecting the
content "tids" pointing to from updating.

This behavior can be easily proved by the compiler. If we add a line of
fake code in the function:
```
tids[0].ip_posid = 0;
```

With current "const ItemPointer tids", the compiler won't report any
problem. If we change to "const ItemPointerData *tids", the compiler
will raise an error due to the assignment to read-only variable.

I searched over the source tree, and found only one more occurrence in
itemptr_to_uint64(), so I fixed it as well.

I have committed this, and I also found a few more similarly confused
cases across the tree, which I also fixed.

Also, as I am touching tbm_add_tuples(), I did a tiny change that moved
the loop variable "i" into "for". Peter Eisentraut just did the same
change in formatting.c [1].

I don't know, let's leave unrelated changes for a separate patch.