Problem

A schema folds into the collection name as schema.name, so a '.' written by the
user collides with the extension's own separator. Two distinct table qualifiers
resolve to one collection:

@Table(schema = "a", name = "b")   // collection a.b
@Table(name = "a.b")                         // collection a.b

Hibernate ORM keeps these as two distinct tables and does not object. Both
entities read and write the same collection.

A dotted table name leaks a schema into the entity's generated sequence, i.e:

@Table(name = "a.b") with @GeneratedValue
  table:    schema=null  name=a.b
  sequence: schema=a  name=b_SEQ     sequence document _id becomes "a.b_SEQ"

The table treats a.b as one opaque name with no schema. The sequence derived
from that same table treats a as a schema. That sequence document _id is the same one
produced by an unrelated @SequenceGenerator(schema = "a", sequenceName =
"b_SEQ"), and Hibernate ORM merges the two into a single sequence, so two
unrelated entities can silently allocate identifiers from one counter. See
HIBERNATE-233 for sequence-style identifier generation.

Proposal

Reject a '.' in a table name and in a schema name at boot, in
MongoAdditionalMappingContributor. Two different table qualifiers can then
never resolve to the same collection name, and the implicit sequence name
derived from a table can no longer contain a '.'.

Scope

Reject:
* a '.' in a table name, including secondary, join, and collection tables
* a '.' in a schema name, whether from the schema attribute or from
hibernate.default_schema

Remove:
* the resolved-name collision check, which exists only to catch this

Compatibility

This removes a currently supported and tested capability: a collection name
containing a '.' can no longer be written directly as a table name.

Collection names with exactly one '.' are still allowed through the schema
attribute, which is the same physical collection:

@Table\(name = "archive.entries"\)               // before
@Table\(schema = "archive", name = "entries"\)   // after, same collection

Collection names with two or more dots become unmappable.

Documentation

Note that a '.' is not permitted in a table or schema name, and that a
schema-qualified entity maps to the collection named schema.name.