fix: register MySQL UDFs once per database

[weicao]

Summary

• register the mysql_rand and mysql_random_bytes DuckDB scalar UDFs once per shared database catalog instead of once per pooled physical connection
• reset the one-time registration state when the connection pool switches to a fresh DuckDB connector
• cover multiple physical connections, connection churn, pool reset, both UDFs, and the first MySQL query after a real server start

Reproduction

On exact main 9767800a, the new multi-connection regression failed on the second physical connection with:

register mysql_rand: database/sql/driver: API error: could not create scalar UDF

Verification

• go test -race ./catalog -run '^TestConnectionPoolRegistersMySQLUDFsOnce$' -count=1 -v
• go test ./catalog -count=1
• first-query regression repeated three times
• go test ./pgserver -run '^TestIssue280RepeatedConnectMySQLDatabase$' -count=1
• go test ./pgserver -run '^TestIssue341PgAuthRejectsWrongPassword$' -count=1
• go test ./backend ./transpiler -count=1
• go test ./charset ./harness ./flightsqltest -count=1
• go vet ./catalog
• go build ./...

[leon-ape]

Independent source review of exact signed head 1babe03 (parent 9767800, tree d29c8bf7c003aa9876b3ab4ca62603d60477218c): no findings. The per-pool sync.Once matches DuckDB catalog-level UDF lifetime, Reset reinitializes the state for a fresh connector, and regression coverage exercises multiple physical connections, churn, reset, both UDFs, and the first real MySQL query.