fix(server-core): correct return type of DialectFactoryFn and ExternalDialectFactoryFn

[AdvMaple]

Problem

Writing a custom Cube.js dialect produces a TypeScript error with no clean fix.

DialectFactoryFn is typed to return a BaseQuery instance, but at runtime Cube.js calls the return value with new. So a correct factory like (ctx) => HanaQuery fails type-checking, and the only escape hatch is:

dialectFactory: (ctx) => HanaQuery as unknown as BaseQuery

That cast is wrong — and misleading to readers.

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Root Cause

#1997 tightened the type from any to BaseQuery, which was good — but BaseQuery and typeof BaseQuery mean different things:

Type	Meaning
BaseQuery	An instance of the class
typeof BaseQuery	The class itself — callable with new

The factory must return a constructor, not an instance.

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Context

Discovered while vendoring a local SAP HANA driver (the original PR #5648 was never merged). dialectFactory: (ctx) => HanaQuery is the correct pattern — it matches every built-in dialect — but it triggers the error above.

BaseQuery has 295+ instance properties populated at construction time by the framework. A custom dialect author cannot satisfy that shape directly; extends BaseQuery is the right approach. The error is purely a type annotation bug.

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Fix

// Before
export type DialectFactoryFn         = (context: DialectContext) => BaseQuery;
export type ExternalDialectFactoryFn = (context: RequestContext) => BaseQuery;

// After
export type DialectFactoryFn         = (context: DialectContext) => typeof BaseQuery;
export type ExternalDialectFactoryFn = (context: RequestContext) => typeof BaseQuery;

With this change, dialectFactory: (ctx) => MyCustomQuery type-checks naturally for any MyCustomQuery extends BaseQuery — no casts needed.

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Testing

Type-only change. No runtime behaviour altered. TypeScript compilation of packages/cubejs-server-core passes.