fix(schema): reject msgpack struct tag on non-PostgreSQL dialects

[kyungseopk1m]

Summary

The ,msgpack struct tag always encodes the value as a Postgres bytea hex literal (\x...) through the hardcoded hex encoder, regardless of dialect. Only Postgres reads that literal back, so on SQLite and MySQL the value is stored as plain text and fails on scan with msgpack: unexpected code=5c decoding map length — the \ (0x5c) at the start of the literal. Same root cause as #519.

#519 was closed with the maintainer noting that msgpack is optimized for Postgres and that other databases should use a custom type. So instead of trying to make msgpack round-trip on every dialect, this returns an error when the tag is used on a non-Postgres dialect. The insert now fails fast instead of silently storing a value that can't be read back. The Postgres path is left untouched.

Reproduction

type Item struct {
	Something int `msgpack:"something"`
}

type Model struct {
	bun.BaseModel `bun:"table:models"`

	ID      int  `bun:",pk,autoincrement"`
	Encoded Item `bun:",msgpack"`
}

// sqlite: insert succeeds, select fails with
// "msgpack: unexpected code=5c decoding map length"

Fixes #1219