A word or phrase that describes a person who is a corporate employee with a very demanding work schedule, but keeps positive and optimistic

Perhaps the 打工人 is a stoic. The Wikipedia entry on Stoicism describes this Hellenistic philosophy:

According to its teachings, as social beings, the path to eudaimonia (happiness, or blessedness) is found in accepting the moment as it presents itself, by not allowing oneself to be controlled by the desire for pleasure or by the fear of pain, by using one's mind to understand the world and to do one's part in nature's plan, and by working together and treating others fairly and justly.

Eudaimonia is:

a Greek word commonly translated as 'happiness' or 'welfare'; however, more accurate translations have been proposed to be 'human flourishing, prosperity' and 'blessedness'.

The stoic will therefore "soldier on" through pain, and is accorded respect by his peers.

I cannot think of one word that covers your definition. There are two that almost do the job and, if taken together, cover it well.

Stakhanovite = An exceptionally hard-working or zealous person

Lexico

it may be an adjective: of or relating to a Stakhanovite or to Stakhanovism.

dictionary online

Pollyanna = a person characterized by irrepressible optimism and a tendency to find good in everything

Merriam Webster

Pollyannaism = blind or excessive optimism, after the character Pollyanna, created by American writer Eleanor Porter (1868-1920).

Free Dictionary

If I had to choose one of these it would be a Pollyanna, because the word certainly covers the unquestioning optimism, and the tendency "to find good in everything" would also apply to hard work. Or a Stakanovite Pollyanna would be such a person as you define.

It sounds very American, for example the smile a woman over the counter at McDonalds gives you when you buy a breakfast McMuffin and when she's on minimum wage (read poverty wages). I'd call it wage slavery. Or ironically, McOptimism (at a McJob).