[codex] Fix ClickHouse quote escaping

[ruslandoga]

What changed

• Escapes embedded double quotes, backticks, and backslashes when rendering quoted ClickHouse identifiers.
• Keeps single-quoted string/comment escaping on the existing ClickHouse-safe '' and \\ path.
• Adds focused SQL generation coverage for string constants, quoted identifiers, backtick identifiers, and table comments.
• Updates the changelog for the current release section.

Why

ClickHouse supports string literals only in single quotes, while identifiers can be quoted with double quotes or backticks. The adapter already escaped string literals, but quoted identifiers were only wrapped, so names containing quote characters or backslashes could render invalid SQL.

Validation

• mix compile
• MIX_ENV=test mix run .context/validate_escaping.exs

mix test test/ecto/adapters/clickhouse/connection_test.exs could not complete in this local workspace because test_helper connects to local ClickHouse and the server rejected the default credentials.