fix(clickhouse): parse SHOW and EXPLAIN without command fallback

[RedZapdos123]

Description:

Issue #7548 reported that ClickHouse SHOW and EXPLAIN statements were parsed as Command fallback nodes, which made them indistinguishable from unknown/unsafe commands.

This PR updates the ClickHouse dialect to:

• parse SHOW TABLES and SHOW CREATE TABLE ... as exp.Show
• parse EXPLAIN ... and EXPLAIN ESTIMATE ... as non-Command AST nodes (exp.Describe with kind=EXPLAIN)
• generate ClickHouse SQL for these ASTs so round-trip behavior is preserved

It also adds regression tests that cover the reported statements.

Closes #7548.

Checklist:

• I have reviewed all changes in this PR myself.
• I have added relevant regression test cases for this fix.
• I have run linting and formatting checks in WSL using python -m ruff check sqlglot/dialects/clickhouse.py sqlglot/parsers/clickhouse.py sqlglot/generators/clickhouse.py tests/dialects/test_clickhouse.py and python -m ruff format --check sqlglot/dialects/clickhouse.py sqlglot/parsers/clickhouse.py sqlglot/generators/clickhouse.py tests/dialects/test_clickhouse.py.
• I have run focused tests in WSL using python -m unittest tests.dialects.test_clickhouse.

The validation screenshot of the tests run, locally on WSL: