fix(dialect): treat a CTE with no main statement as a degraded parse (#698)

[Yuren0312]

Summary

Fixes #698 — valid multi-CTE queries that use a map/array subscript such as
m['key'] or ARRAY_JOIN(m['key'], ' ') raise
InvalidQueryDefinition("WITH clause without a main statement is not valid SQL").

Root cause

When the SQL contains a [ (map/array element access), DialectParser._detect_dialects
selects the TSQL BracketedTableDialect first. Under that dialect sqlglot's WARN-mode
parse misreads the [ as a bracket-quoted identifier and silently drops the trailing
main SELECT, leaving a bare top-level exp.With node.

DialectParser._is_degraded only checked for an opaque exp.Command or bad
table/column names, so this broken parse was accepted as the winning result — and the
missing main statement surfaced later as InvalidQueryDefinition when .tables/.columns
were accessed.

Change

DialectParser._is_degraded now also returns True when the root AST node is a
top-level exp.With (a CTE with no main statement). A complete query always parses to
its main statement (Select/Insert/…) with the CTE attached as a with argument, so a
bare top-level With is unambiguously a degraded result. _try_dialects then falls
through to the next candidate dialect, which parses the query correctly.

Verification

• Added a regression test (test_with_map_subscript_in_cte_does_not_drop_main_statement)
  covering ARRAY_JOIN(m['k'], ' '), a plain m['k'] subscript, and an arr[1] subscript
  inside a non-first CTE. All now return the correct tables instead of raising.
• Full suite: 270 passed, with the only 4 failures being pre-existing and unrelated
  (they fail identically on the unmodified code — test_comments.py / test_query_type.py
  DROP/DELETE-with-comment cases, caused by sqlglot version drift, not this change).