feat: emit warning diagnostic when comparing with NULL using = or <>

[wilbyang]

Which issue does this PR close?

Closes #14434

Rationale for this change

expr = NULL and expr <> NULL always evaluate to NULL (never TRUE or FALSE) due to SQL's three-valued logic. This is almost always a bug — the user almost certainly meant IS NULL or IS NOT NULL. DataFusion currently executes these queries silently with no feedback.

What changes are included in this PR?

Warning infrastructure (datafusion/sql/src/planner.rs)

Added warnings: Arc<Mutex<Vec<Diagnostic>>> to PlannerContext with two new public methods:

• add_warning(diag: Diagnostic) — push a non-fatal warning during planning
• take_warnings() -> Vec<Diagnostic> — drain all collected warnings after planning

Using Arc ensures all clones of PlannerContext (e.g. for subqueries) share the same backing store, so warnings from nested queries surface to the top-level caller. This is the first use of Diagnostic::new_warning in the codebase and lays the foundation for future non-fatal diagnostics.

Detection (datafusion/sql/src/expr/mod.rs)

In the binary expression stack machine, detects = NULL / <> NULL on either operand and emits a Diagnostic::new_warning:

'= NULL' will always be NULL (null comparisons never return true or false)
help: use 'IS NULL' instead

The span points at the non-null operand (the column/expression), since NULL literals carry no span.

Tests (datafusion/sql/tests/cases/diagnostic.rs)

Added a do_query_warnings helper and 5 new tests:

Test	Expectation
WHERE col = NULL	1 warning, span on col, suggests IS NULL
WHERE NULL = col	1 warning, span on col, suggests IS NULL
WHERE col <> NULL	1 warning, span on col, suggests IS NOT NULL
WHERE col IS NULL	0 warnings (correct form)
WHERE a = NULL AND b = NULL	2 warnings

How callers consume warnings

let mut planner_context = PlannerContext::new();
let plan = sql_to_rel
    .sql_statement_to_plan_with_context(stmt, &mut planner_context)?;
let warnings = planner_context.take_warnings(); // Vec<Diagnostic>

Are these changes tested?

Yes — 5 new tests, all passing. Full datafusion-sql test suite passes (459 + 69 + 12 tests).