fix: skip multi-statement SET validation in session mode

[cschanhniem]

In session mode, PgDog should forward multi-statement queries to the server verbatim without validating whether they mix SET with other commands. The server owns the session and can handle mixed statements natively.

Root cause: try_multi_set() unconditionally rejects any multi-statement query that mixes SET with non-SET statements (e.g., SHOW, SELECT). In session mode this validation is incorrect: there is no logical connection to pool, so mixed SET + other statements are perfectly valid.

Fix: When the pool is in session mode, try_multi_set() returns Ok(None), causing the query to fall through to normal processing and be forwarded to the server as-is.

Example query that was previously rejected but now works:

SET DateStyle='ISO'; SET extra_float_digits = 2; show transaction_isolation

Closed issues: Closes #1087

[cschanhniem]

@CLAassistant signed

[levkk]

Not sure what happened here. I think GitHub has yet another bug. I don't think I merged this and it doesn't appear in the git log.

[DirtyRacer1337]

it's right here, you can cherry pick it, 9235234

still actual problem for me

[levkk]

Could you please resubmit the PR? Thank you and sorry for the trouble.

[DirtyRacer1337]

done #1119