feat: Check partition existence and health in divider before executing table creation

[soniapi]

When divider is called to create partition tables based on the divider_value, it currently blindly executes the CREATE TABLE queries which will fail if the tables were previously created (e.g. by manual migration). This change queries pg_tables in the public schema to verify if both tables (objects_s_below_... and objects_s_above_...) exist, runs a quick SELECT 1 FROM <partition> LIMIT 1; health check to ensure they are queryable, and if everything is correct, logs an informative message and bypasses the CREATE TABLE statement execution.

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PR created automatically by Jules for task 13430159607150436215 started by @soniapi

[google-labs-jules]

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