feat: replace regex+macro MV external target with config.get_rendered

I have closed https://github.com/ClickHouse/dbt-clickhouse/pull/645 as it can't be developed until https://github.com/dbt-labs/dbt-core/pull/12965 is merged. Keeping this issue open to track when that PR get merged so we can definetively work on it.

# TL, DR of the PR:

Replaces the `materialization_target_table()` macro + SQL comment regex approach for external-target materialized views with `config.get_rendered('target_table')`, a clean dbt-native config key.

### Before

Users had to call a macro that injected a SQL comment, which the materialization then regex-parsed:

```sql
{{ config(materialized='materialized_view') }}
{{ materialization_target_table(ref('my_target')) }} {# outputs: -- materialization_target_table: schema.table #}
SELECT ...
```

This was incompatible with dbt-fusion (which does static DAG analysis, not SQL comment parsing) and required the regex:
```jinja
{%- set target_match = modules.re.search('--\\s*materialization_target_table:\\s*(.+?)\\s*$', sql, modules.re.MULTILINE) -%}
```

### After

```sql
{{ config(
materialized='materialized_view',
target_table=ref('my_target')
) }}
SELECT ...
```

The `target_table` value is a `RelationProxy` captured at render time via `config.get_rendered('target_table')` and used directly in the `TO` clause — no comment injection, no regex, no `| string` coercion.

Wait for dbt-core PR #12965 to merge before starting. Then inspect the materialized_view implementation and the materialization_target_table() macro path, replacing the SQL-comment regex approach with the target_table config and config.get_rendered('target_table') flow. Done means the target is used directly in the TO clause without comment injection or regex parsing.

Written by the indexing model from the issue text.