Normally, `MakeTenantPrefix(ID).PrefixEnd()` is the same as
`MakeTenantPrefix(ID + 1)`. However, this isn't true if the ID
corresponds to one of the boundary points in `EncodeUvarintAscending`.
Prior to this patch, various places in the code did not appreciate this
subtlety. This proved problematic when we started creating split points
at `MakeTenantPrefix(ID).PrefixEnd()`. Now, if the tenant ID was equal
to one of these boundary points (e.g. 109), decoding the key
`MakeTenantPrefix(ID).PrefixEnd()` does not return the expected tenant
ID (in our case, 110). Instead, it results in an error.

Worse yet, various places in KV assume doing such tenant decoding at a
range's start key is kosher. We've since disallowed KV from accepting
such splits in cockroachdb#104802.
This patch fixes the root cause of why these splits were being issued
in the first place -- over in
cockroachdb@ac54eba,
we started creating spilt points at the end of a tenant's keyspace
(in addition to the start of it). We did so using `PrefixEnd()`, which
we have since learned is not what we wanted here.

In addition to the initial split point, we also fix the span config
records we seed during tenant creation. We've also added a test to
ensure all split points created during cluster creation are kosher.
The test uses randomized tenant IDs -- I confirmed that it fails with
tenant ID = 109 (without my changes).

Lastly, there's a bit more auditing work that needs to be done here
about these assumptions. That's captured in a followup issue
cockroachdb#104928.

Prevents cockroachdb#104606 from
happening.

Release note (bug fix): Fixes a bug where tenant creation for certain
IDs would always fail because of invalid split points. Additionally,
such tenant creation failure could leave the host cluster's span
config state entirely busted -- we prevent that as well.