fix(signature): realigning SignatureManager with specs

[guzmud]

Description

Following drift in specs and parallel development, SignatureManager and related elements must be updated to realign between the OpenAEV backend and the OpenAEV injectors.

1. The url to be targeted is supposed to be /api/injects/execution/callback/{injectId} instead of /api/injects/{injectId}/callback
2. The payload should be aligned with feat(signatures): implement structured output processor behaviour in backend openaev#5447, other sources of truth were out-of-date

Additional information

This is a follow-up to #206 and #257

[Filigran-Automation]

🤖 [AI-generated]

Hey @guzmud! 👋 Thanks a lot for opening issue #284 and for pinpointing the wrong vs. correct path so precisely — that's really helpful! 🙏

I just had a quick look and I think the description could be enhanced a little to help maintainers act on it faster. I haven't changed anything in your description — just a gentle, optional suggestion:

Area	What could help	Suggestion
## Environment	Placeholders left in ({ e.g. macOS 14 }, { e.g. 1.2.3 })	Fill in the client-python version where the wrong path is hit
## Reproducible steps	Template placeholders ({ e.g. Run ... })	Add the minimal steps that trigger the call to the wrong endpoint
## Expected output / ## Actual output	Both empty	Note that the request should hit /api/injects/execution/callback/{injectId} but currently goes to /api/injects/{injectId}/callback (e.g. resulting status/error)

💡 If helpful, the contribution conventions and the bug issue template walk through what to include.

No rush at all — thanks again for contributing to OpenAEV! 🚀