[php-src] PR #23892: Fix getColumnMeta() for GH-15287 Pdo\Pgsql::setAttribute(PDO::ATTR_PREFETCH, 0)

Pull Request: https://github.com/php/php-src/pull/23892 Author: outtersg As stated in the UPGRADING, using the passthrough ("single-row") mode of libpq (introduced in https://github.com/php/php-src/issues/15287) forbids passing a new query while the current one's results have not been entirely consumed. … But I didn't notice that ext/pdo_pgsql internally used new queries to fetch metadata (example use case: a call to getColumnMeta() while fetch()ing row by row will interleave the getColumnMeta()-triggered internal query to the database, with the results fetching for the user-called query). This PR: * **prevents getColumnMeta() from interfering with an unbuffered user query**\ by returning an empty value for every metadata that would emit an internal query, thus breaking libpq inner workings * caches the table name so that **30 calls to getColumnMeta() won't call the database 30 times** This is a second take to #16249; compared to it: * **+** The [UAF](https://github.com/php/php-src/pull/16249#discussion_r4095582864) is fixed * **+** The [egg-and-chicken problem](https://github.com/php/php-src/pull/16249#issuecomment-3349120535) at shutdown time may hopefully be fixed\ thanks to a rebasing on the latest master which includes fixes from #23065 (and maybe #17567 too?) * **-** I'm giving up on generalizing the PR to every internal query, focusing on getColumnMeta() only:\ This can be done on a call-by-call basis, as did in fact @KentarouTakeda for COPY in the aforementioned PR Of course I'd feel more comfortable if there was a way to run the full CI chain _before_ merging: any hint from GH rgulars?