Feat/lsp fixes

[sdairs]

• Formatter upper-cased identifiers that matched keywords. t.type, AS
  type, system.type were emitted as t.TYPE, AS TYPE, SYSTEM.TYPE — which
  changes the query's meaning or requires backticks that the user didn't
  write. Track an identifier_depth in FormatterContext; BareWords inside
  TableIdentifier / ColumnReference / QualifiedName / the name slot of
  TableAlias/ColumnAlias are never re-cased. Clause-keyword positions are
  unaffected.
• Dot-access missed when cursor sat at a token boundary. Live-typing
  SELECT t. with FROM adjacent produced SELECT t.|FROM …;
  token_before_offset used token.start <= offset so it picked the next token
  as "prev" and completion fired as a plain select expression. Switched to
  token.end <= offset in token_before_impl and find_ident_before_dot_impl.
• SELECT clause swallowed adjacent clause keywords. SELECT t.|FROM x was
  parsed as a qualified column t.FROM, destroying the FROM clause and
  blanking the scope. Added select::nth_is_clause_keyword(p, n); guarded
  both the identifier chain in expr_delimited and the postfix Dot branch in
  parse_expression_rec.
• No columns inside subqueries, no projection through subquery aliases.
  scope_at now uses find_enclosing_statement, so a cursor inside a subquery
  gets the subquery's own scope. Added QueryScope::subquery_refs; when a
  TableAlias follows a SubqueryExpression, we record (alias, columns) with
  columns taken from the subquery's top-level SELECT list (AS-aliases
  preferred, trailing identifier otherwise). Completion surfaces those
  columns directly.
• JSON(..., SKIP REGEXP 'pattern', ...) flagged a red squiggle. The SKIP
  handler accepted SKIP path and SKIP 'str' but not the three-token SKIP
  REGEXP 'pat' form, so REGEXP was consumed as the path and the string was
  left stray. Match the REGEXP keyword specifically and require a string
  literal after it.
• CREATE TABLE … CLONE AS source flagged Unexpected token in CREATE
  statement. Added the Clone keyword and a CLONE AS branch in
  parse_create_table that wraps the source as an AsClause and falls through
  to standard ENGINE = … / ORDER BY … handling for optional overrides.
• Connected server posted "Only SELECT is supported for EXPLAIN query" on every non-SELECT statement. publish_diagnostics validated anything whose source text started with SELECT/WITH/INSERT by running EXPLAIN PLAN, but ClickHouse
  only accepts EXPLAIN for SELECT (and UNION of SELECTs). Every INSERT/CREATE/ALTER/etc. in a connected workspace got a yellow squiggle with a server error the user couldn't act on. Filter on the CST statement kind (SelectStatement
  or UnionClause) instead of text prefixes.
• Fix clippy test failing to run
• Fix all clippy warning