minor: validate config default_null_ordering when setting it

[shinzoxD]

Which issue does this PR close?

• Part of The config values aren't checked #17498

Rationale for this change

datafusion.sql_parser.default_null_ordering is documented to accept only four values (nulls_max, nulls_min, nulls_first, nulls_last), but it was stored as a raw String. Invalid values were accepted at SET time and later converted with NullOrdering's From<&str> impl, which silently fell back to nulls_max.

For example:

> SET datafusion.sql_parser.default_null_ordering = nuls_max;
0 row(s) fetched.

That typo was stored and then treated as nulls_max, with no hint that the value was invalid. This is the same class of problem as explain.format (fixed in #17549) and the later per-option validation PRs on #17498.

What changes are included in this PR?

• Add a typed NullOrdering config enum in datafusion-common that validates values when the setting is applied.
• Change SqlParserOptions::default_null_ordering from String to NullOrdering.
• Re-export that enum from datafusion-sql so existing datafusion_sql::planner::NullOrdering users keep compiling.
• Reject invalid values immediately, including the previously accepted empty string.
• Accept the four documented values case-insensitively, matching other enum config options.

Are these changes tested?

Yes.

• Unit test for valid values, case-insensitive parsing, invalid values, and leaving the previous setting unchanged on error
• set_variable.slt coverage for an invalid SET
• order.slt now expects empty / typo values to error instead of silently using nulls_max
• cargo test -p datafusion-common test_default_null_ordering_validation
• cargo test -p datafusion-sql --lib
• cargo test -p datafusion-sql --test sql_integration
• cargo test -p datafusion-sqllogictest --test sqllogictests -- -- set_variable.slt
• cargo clippy -p datafusion-common -p datafusion-sql -p datafusion --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check

Are there any user-facing changes?

Yes.

• SET datafusion.sql_parser.default_null_ordering now errors on invalid values instead of silently using nulls_max.
• SqlParserOptions::default_null_ordering is now NullOrdering rather than String (API change).
• datafusion_sql::planner::NullOrdering is a re-export of the common type. From<&str> (the silent fallback) is removed; parse with FromStr instead.

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[shinzoxD]

Closing as a duplicate of #24400 — same default_null_ordering validation, opened first. Please review #24400.