Require `TransactionTrait::Transaction` to be a fixed point (#3153)

The methods generated by `#[sea_orm::model]` (`insert`/`update`/`save`/
`delete`/`action`/`cascade_delete`) open a transaction and then save related
models inside it, so `action::<C>` recurses into `action::<C::Transaction>`.
Proving those futures `Send` without a concrete `C` walked the unbounded
`C::Transaction::Transaction::...` chain and overflowed (E0275 /
`clippy::future_not_send`), so the documented 2.0 entity format failed to
compile under clippy.

Fix it on the trait rather than on the generated methods: `TransactionTrait`
now requires `Sync` and pins its associated `Transaction` to a fixed point
(`Transaction::Transaction == Transaction`) that is `Send`. That collapses the
chain to a single type, so the obligation terminates.

`Sync` and `Send` exclude no implementor: the `#[async_trait]` desugaring
already forces both, since `begin` holds `&self` across an await and
`TransactionSession::commit` takes `self` by value. Only a non-fixed-point
`Transaction` is newly rejected, and it errors at the offending `impl` rather
than at `#[sea_orm::model]`. All in-crate implementors are fixed points already.

Putting the bounds on the generated methods instead (as in #3149) fixes the
same overflow but leaks into user call sites: a helper generic over the
connection, e.g. `fn create<C: ConnectionTrait + TransactionTrait>(db: &C)`,
compiles on 2.0.0 and stops compiling, with no exported alias to restate. On
the trait the requirement is invisible to callers.

The regression test is async-only (the sync crate has no futures to prove
`Send`), so it lives in the main crate's `tests/` gated on `feature =
"sqlx-dep"` and compiles out in `sea-orm-sync`. Its `assert_send` calls are the
actual checks, covering every generated method generically over the connection
across has_many / has_one / belongs_to / many-to-many / self-ref / no-relation
shapes, the `ConnectionTrait + TransactionTrait` and `impl Trait` caller forms,
the concrete connection types, and a downstream `TransactionTrait` impl. Without
the fix it fails with 7 E0275, checked by plain `cargo test`.