Fix memoize re-invoking factories that return undefined

[Montana]

Problem

memoize() uses typeof memo !== "undefined" as its cache-hit check, so any factory whose result is undefined is silently re-invoked on every container.get() call. This breaks the documented run-once/singleton guarantee — notably for the side-effect initializer pattern the docs recommend with run() (e.g. Injectable("initCache", ["request"], (req) => populateCache(req)), whose factory returns nothing and therefore re-runs on every subsequent resolution).

Repro:

let calls = 0;
const c = Container.provides("init", () => { calls++; return undefined; });
c.get("init"); c.get("init"); c.get("init");
// calls === 3, expected 1

Fix

Track invocation with an explicit invoked flag instead of inspecting the memoized value. The flag is set only after the delegate returns, deliberately preserving the existing retry-on-throw behavior that existing tests rely on.

Tests

• New memoize.spec.ts: undefined-result caching, throw-then-retry, delegate/isMemoized behavior
• Container-level regression test in Container.spec.ts

All tests pass with 100% coverage maintained; ESLint clean.

[kburov-sc]

LGTM, thank you for the contribution

[Montana]

@kburov-sc , No problem. I'm glad to help anytime.

[RobbieBizzle]

@Montana the hero!