Hp damage is applied as one big lump, not point-by-point. Taking 50 damage is not the same as taking 1 damage 50 times—and you can entirely skip the “dying” stage if you get hit with something big enough. In fact, this is a big part of the reason why most effects that trigger off of the “dying” state are considered so weak—since they have a window of just 9 hp for you to actually be dying, and mid-to-high-level effects deal dozens or even hundreds of damage, odds are very good you’ll never actually be “dying,” you’ll just go straight from “fine” to “dead.”
This also means that there is no question of healing “interrupting” damage—it can’t happen in the middle of a chunk of damage. In any event, there certainly isn’t any rule that healing protects against future or ongoing damage—heal might undo the condition causing the damage, but otherwise you’re still going to take it.
And you can still take damage well below −10 hp. The dead condition reads “Dead (−10 Hit Points or Lower),” which wouldn’t be necessary if you just stopped taking damage at −10 hp. Most things that deal damage probably won’t damage a dead creature (though this is debatable), but whatever overkill there was in the effect that killed you still applied. So even if you had some effect that could heal hp to a dead creature, and revive that creature if the healing was sufficient to bring them above −10 hp, your scenario has you starting from −900 hp—it’s going to take a lot more than fortunate fate to get you out of that one.
Bottom line, none of these effects is remotely sufficient to save you from 1000 damage.
But as for what actually happens, that gets trickier.
The contingency spell is broken for many reasons, but one big one is the nearly complete and utter lack of limitations on the trigger. But one that I’ll stand by is that it cannot predict the future—and a spell triggered after you have died will not help you, since heal cannot restore hp to a dead target.
Likewise, stalwart pact’s temporary hp don’t apply to a dead target, and its wording makes it clear that its effect only applies after the damage has been dealt. So an attack that takes you from above half hp to dead will bypass stalwart pact.
But then we have fortunate fate—and that one explicitly says it triggers if “an effect would kill [the subject].” The word “would” here indicates that fortunate fate is triggered prior to your actual death. So you gain 150 hp, and then take damage from whatever would have killed you. If it’s still enough to kill you, you’re dead. If it’s enough to save you, but only just barely—leaving you dying—then your contingency should probably trigger (though a DM might rule that a trigger worded with a future condition cannot trigger at all as contingency will always miss the trigger). And if you’re below half hp, stalwart pact can trigger, giving you temporary hp.
It’s not at all clear under the rules if stalwart pact and your contingency interfere with one another. If both are triggered, but then one undoes the condition that triggered the other (stalwart pact’s thp makes you no longer dying, heal brings you above half your hp), does the other triggered effect still take place? No one knows. For that matter, the order of effects isn’t really defined either—there’s a guideline that the controller of simultaneous effects should determine their order, but it’s only a guideline. Anyway, if the heal effect applies first, and leaves you still below half hp, stalwart pact’s condition is still met and that’s undoubtedly still triggered and applies. In that scenario you definitely get all of the effects, it’s just unclear if that scenario is possible, or what happens in any of the alternative scenarios.
Finally, note that a potentially-better trigger for contingency in this situation is something like “if fortunate fate just healed me, and I am still missing \$X\$ hp,” so that you can be healed some more. This maximizes the potential of the effects to keep you alive by making all of the healing happen before damage is dealt, and allows contingency to “piggy-back” off of fortunate fate’s limited ability to predict the future, instead of relying on contingency itself having that ability.
The other thing to keep in mind is that there may be better things to do than attempting to heal yourself. The problem is that your healing either occurs before the damage—which only helps if you’re already missing hp, and the missing hp is enough to make the difference—or after, which does no good if the damage kills you. This is a problem because there’s a whole lot of ways to pump damage well past one-shot-kill thresholds, making the healing pointless either way. A contingency to move you outside the effect or apply a relevant immunity would be much better. Damage prevented is vastly better than damage healed.
The other approach would be to use contingency to cast revivify. The problem with that approach is that it leaves you rather-vulnerable. I recommend not triggering revivify on death, but rather right before your turn would have happened if you didn’t die—that gives you a turn to heal or get out of the situation.
