Parse the MySQL address with net.SplitHostPort

[arpitjain099]

ConfigFromMySQL splits dbConfig.Addr on ":" and indexes element 1:

addr := strings.Split(dbConfig.Addr, ":")
dc.Host = addr[0]
port, err := strconv.Atoi(addr[1])

go-sql-driver's ParseDSN only fills in a default port when Net is "tcp". For a unix socket DSN it leaves Addr as a bare socket path with no colon, so there is no element 1:

user:pass@tcp(127.0.0.1:3306)/testify      -> Host="127.0.0.1" Port=3306
user:pass@unix(/tmp/mysql.sock)/testify    -> PANIC: runtime error: index out of range [1] with length 1
user:pass@tcp([::1]:3306)/testify          -> err=could not parse mysql port: strconv.Atoi: parsing "": invalid syntax

Both production callers reach it with the configured connection string, NewEntClient in client.go and RewriteConnectionStringForIAM in iam.go, so a socket DSN takes the server down at startup rather than reporting a bad configuration.

The third line is a second, quieter problem from the same split: a bracketed IPv6 host leaves Host as "[" and the port empty.

net.SplitHostPort handles both shapes, and a non-tcp network is now an explicit error that says what is supported rather than a panic.

Added utils_test.go covering tcp, IPv6, the unix socket, and an unparseable DSN. Without the change the IPv6 and unix cases both fail, the latter by panicking.

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