[elixir] feat: Allow comment and custom_properties on create_database

[nicolazar]

Purpose

Linked issue: close #587

Brief change log

This PR allows callers to set a database comment and custom_properties when creating a database, via an optional Fluss.DatabaseDescriptor:

descriptor =
  Fluss.DatabaseDescriptor.new()
  |> Fluss.DatabaseDescriptor.comment("App data")
  |> Fluss.DatabaseDescriptor.put_custom_property("owner", "team-x")

:ok = Fluss.Admin.create_database(admin, "my_db", descriptor)

Previously admin_create_database hard-coded the descriptor to None, so the only way to observe a descriptor was to read one back via get_database_info/2 (#586). This closes that gap on the write side.

We now change the create_database function to accept a descriptor as the third positional argument, before ignore_if_exists.

Tests

Added unit tests for the builder helpers (happy paths, overwrites, cross-field preservation, guard/validation failures) and integration tests for create_database/3 with a descriptor (comment-only, properties-only, struct-directly), asserting the values round-trip through get_database_info/2.

Additionally, strengthened the existing get_database_info/2 test to assert descriptor values rather than just shape.

API and Format

The signature of admin create_database/3 function has been changed: previously it meant create_database(admin, name, ignore_if_exists). It now means create_database(admin, name, descriptor). Callers that passed ignore_if_exists positionally as the third argument must move it to the fourth. The three in-repo callers have been updated.