sql: validate recursive CTE references in subqueries and outer joins

[shashi1973]

Fixes: #136623

This change adds PostgreSQL-compatible validation for recursive CTE
references in contexts where PostgreSQL does not allow them.

Specifically, it rejects recursive references that occur:

• inside a subquery
• on the null-extended side of a LEFT, RIGHT, or FULL OUTER JOIN

The validation tracks the subquery and outer-join context relative to
the start of the recursive term. This avoids rejecting a recursive CTE
merely because the CTE itself appears inside an enclosing subquery.

Tests cover:

• recursive references inside subqueries
• recursive references on the nullable side of LEFT JOIN
• recursive references on the nullable side of RIGHT JOIN
• recursive references on the nullable sides of FULL JOIN
• a positive case verifying that a recursive CTE inside an enclosing
  subquery remains valid when the recursive reference itself is not
  inside another subquery

Testing:

./dev test pkg/sql/opt/optbuilder --filter=TestBuilder

./dev generate

./dev lint --short was also run. The lint run was blocked by the
existing CODEOWNERS check reporting stale paths **.pb.go and
**.pb.gw.go; no lint failure was reported for the files changed by
this PR.

Release note (bug fix): Recursive CTE references in subqueries and on
the null-extended side of outer joins are now rejected to match
PostgreSQL behavior.

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