Type EmptyFilter as a string literal instead of a unique symbol

[candrewlee14]

Stacked on #6200 — it contains that PR's commits until it lands. Review only the last commit, Type EmptyFilter as a string literal.

Separate from #6200 because it is the one change that touches observable API. That call is yours; #6200 stands on its own without it.

What

-export const EmptyFilter = Symbol.for('drizzle:EmptyFilter');
+export const EmptyFilter = 'drizzle:EmptyFilter';
 export type EmptyFilter = typeof EmptyFilter;

Why

#6200 strips every nominal member from the dialects — including AggregatedField#table, which is what the recursive selection type was actually tripping over — and all three then come to rest here. I verified this is still load-bearing after that: with #6200 applied and this change reverted, all three dialects fail again on Type 'unique symbol' is not assignable to type 'AnyTableFilter | unique symbol'.

Symbol.for returns the same symbol in every copy of the package on disk, so the value was already portable at runtime — but its inferred type is unique symbol, which is nominal per declaration site. The type contradicted the runtime, and every public type mentioning EmptyFilter stopped being assignable across installs.

Why a string literal specifically

The filter builder relies on unit-ness:

if (processed === EmptyFilter) continue;
parts.push(processed);   // <- requires EmptyFilter to be narrowed out of the union

Only unit types narrow under ===. I tried the obvious structural alternative first — a branded symbol & { readonly [Brand]: true } — and it fails to compile at exactly that line: structural, but not a unit type, so the guard stops narrowing. A string literal is the only type that is both, so it is the only option that keeps the narrowing and gains the portability.

Trade-off, explicitly

This changes an exported runtime value from a symbol to a string. Identity comparison (where === EmptyFilter, where: EmptyFilter) is unaffected, and EmptyFilter is confined to relations.ts internally — but it is observable: typeof EmptyFilter === 'symbol', or reliance on symbol non-enumerability when serialising a filter, would notice.

If that is unacceptable, drop this PR. It costs the three dialect rows and nothing else.

Result

PgDialect, MySqlDialect and SQLiteDialect flip to portable — 22 of 30 to 25 of 30 on the matrix.

The cockroach, mssql and singlestore dialects need this change too, but do not flip: they reach their session class and are blocked behind a further chain of nominal members. They stay recorded rather than half-fixed.

Verification

Full drizzle-orm suite green (27 files, 1344 tests); test:types clean, including the narrowing site above.