fix: validate connection instance in Model.useConnection() (#16098)

[22BRS1317]

Problem

Model.useConnection() only validated that the connection argument was
not null or undefined, but accepted any non-nullish value (plain objects,
strings, numbers, etc.) without throwing an error. This led to confusing
failures later in the code rather than a clear error at the call site.

Solution

Added an instanceof Connection check immediately after the existing null
check in lib/model.js. If a non-Connection value is passed, a descriptive
MongooseError is thrown with the type and value of the invalid argument.

Changes

• lib/model.js — added instanceof Connection validation in
  Model.useConnection() with a clear error message
• test/model.test.js — added test case inside the existing
  describe('Model.useConnection()') block covering objects,
  strings, numbers, and null

Test

assert.throws(
  () => Model.useConnection({ not: 'a connection' }),
  /must be an instance of mongoose\.Connection/
);

Fixes #16098

[vkarpov15]

Closing in favor of #16149