Normalize finding signature_hex generation

Mehmet Emin INACrequested to merge
minac_fix_signature_hex into master

What does this MR do and why?

Normalize finding signature_hex generation

Hash the canonical signature payload before exposing signature_hex in both CI report and EE model implementations. This keeps comparison and deduplication stable across qualified and unqualified signatures and avoids format-specific raw signature output.

Related to Follow-up from "Add application-layer length va... (#598390).

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