fix(discovery-core): handle open notes properly

[m-kus]

TL;DR

Added support for open notes by introducing an is_open field to DecryptedNote and implementing plaintext amount handling for notes with salt value 1.

What changed?

• Added is_open boolean field to the DecryptedNote struct to indicate whether a note stores its amount in plaintext
• Imported OPEN_NOTE_SALT constant and modified decrypt_note() function to check if salt equals 1 (open note)
• For open notes (salt == 1), the function now returns the plaintext amount directly instead of attempting ECDH-based decryption
• Added comprehensive test coverage for both open note and encrypted note scenarios
• Defined OPEN_NOTE_SALT constant with value 1 in the decryption module
• Removed the TODO comment about handling open notes since this functionality is now implemented

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This change is 

[m-kus]

• fix(discovery-core): handle open notes properly #588 👈 (View in Graphite)
• chore(discovery-service): adjust default config #585
• feat(discovery-service): validate viewing key #566
• main

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[Yoni-Starkware]

@Yoni-Starkware reviewed 3 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on m-kus).