PostgreSQL: support right-deep join chains (deferred ON clauses)

[moshap-firebolt]

PostgreSQL accepts join chains where ON clauses are deferred to the end, e.g.:

t0 JOIN t1 JOIN t2 ON c1 ON c2

This is equivalent to t0 JOIN (t1 JOIN t2 ON c1) ON c2 per the SQL standard's
right-associative interpretation when parentheses are absent. The parser already
handles this for Snowflake via supports_left_associative_joins_without_parens();
PostgreSQL was missing the override, causing a parse error on any right-deep chain
of depth >= 3.

Also filed upstream: apache#2377

cargo test --test sqlparser_postgres passes locally (180 tests).

---

Note

Low Risk
Parser and dialect-flag changes only; scope is SQL join parsing for PostgreSQL with targeted tests and no auth or data-path impact.

Overview
PostgreSQL now opts out of left-associative join parsing so chains like t0 JOIN t1 JOIN t2 ON c1 ON c2 parse and canonicalize to nested joins (e.g. t0 JOIN (t1 JOIN t2 ON c1) ON c2), matching Postgres right-deep join syntax.

The join parser skips auto-wrapping in nested joins when the current join is NATURAL, so NATURAL JOIN followed by a constrained join stays left-associative instead of being parenthesized like Snowflake-style right-deep chains.

Shared join precedence tests are collapsed to one canonical form; new PostgreSQL tests cover multi-table right-deep chains and NATURAL + INNER JOIN.

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