Re: pgstat: allow a stats kind to use its own dedicated dsa/dshash

Hi,

> An automatic claude review pointed out that a stats file hash entry
> for a fixed-numbered kind now crashes during startup.

Good find.

> The attached patch fixes this and includes a simple test case in it to
> show the issue.

A test is good here. I don't think we have enough test coverage for
pgstat startup.

However, I think we could keep the proposed test simpler, and flip the
kind from
variable to fixed once, which is enough.

Also, despite the comments in pgstat_internal.h:

```
/*
* For variable-numbered stats. Optional.
*/
void (*to_serialized_name) (const PgStat_HashKey *key,
const PgStatShared_Common *header, NameData *name);
bool (*from_serialized_name) (const NameData *name, PgStat_HashKey *key);

```

we do not prevent a fixed-numbered kind from defining these callbacks.
I think we should harden that. With it in place, the new check is
only needed for PGSTAT_FILE_ENTRY_HASH, as a name entry for a
fixed-numbered kind will then always fail the !from_serialized_name
check when PGSTAT_FILE_ENTRY_NAME.

0001 disallows the serialized name callbacks for fixed-numbered kinds.
0002 is your fix with the check in the hash entry branch and the
simplified test.

--
Sami