Raw() does not return if connection is closed

Nobody has claimed this yet.

**Describe the bug**
I am accessing a connection via `Raw()` using the stdlib compatibility layer in order to close a connection that is in a bad state. I discovered that if I am in a transaction, `Raw()` does not return, and instead hangs. If I rollback the transaction before calling `Raw()`, this does not happen.

I'm not sure if this is a bug in pgx or maybe `database/sql`.

**To Reproduce**
I put a reproducer here: https://gist.github.com/horgh/e3ba60ecc20f8cf2bf7e4d795c336350

The reproducer includes a `Rollback()` which avoids the hang. If you comment out the `Rollback()` on line 69 you will see it hang. There are a few other comments in there that might be of interest.

Please run your example with the race detector enabled. For example, `go run -race main.go` or `go test -race`.

**Expected behavior**
`Raw()` should return.

**Actual behavior**
`Raw()` does not return

**Version**
- Go: `$ go version` -> go version go1.24.4 linux/amd64
- PostgreSQL: `$ psql --no-psqlrc --tuples-only -c 'select version()'` -> PostgreSQL 16.9 (Ubuntu 16.9-1.pgdg24.04+1) on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0, 64-bit
- pgx: `$ grep 'github.com/jackc/pgx/v[0-9]' go.mod` -> v5.7.5

**Additional context**
I am closing the connection to try to work around #2223

Start with the linked reproducer in main.go, especially the Raw() call and the Rollback() on line 69, then run it with the race detector as requested. Trace the stdlib compatibility layer's transaction behavior and verify that Raw() returns without requiring Rollback() first and without hanging.

Written by the indexing model from the issue text.