Issue #8777: Fix false positive result on error and populate schema in Action Truncate tables

[samuelwillyanto1]

Fixes #8777

Description

1. Fixes false positive result: calls updateErrors() on database connection failure and execution error so result.setResult(false) and nrErrors > 0.
2. Auto-populates both table and schema names in ActionTruncateTablesDialog when fetching table names, and validates database connection before querying.

3. Implements check(...) to warn when no tables are selected for truncation.

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