[电大老baby (HhH666-ux)]

Description

SeedGroup._prompts_to_messages passed the seed's metadata mapping directly to the derived MessagePiece, so nested values were shared between the seed and every extracted message. Mutating a derived message's metadata therefore also changed the source seed and later extractions.

• Deep-copy the seed metadata before assigning it to the new message piece.
• Add a regression test that mutates nested metadata on history and next-message pieces and asserts the source seeds and subsequent extractions stay unchanged.

Fixes #3899

[Copilot]

🟢 Approval recommended

No active review issues are recorded for the change isolating seed metadata from message mutations.

No open issues 🎉

What changed in this PR

Fixes #3899 by isolating metadata when SeedGroup converts seed prompts into messages, preventing nested message metadata mutations from affecting source seeds.

Changes:

• Deep-copy seed metadata at the seed-to-message boundary.
• Add a regression test covering history and next-message mutations, unchanged source seeds, and a subsequent history extraction.

File	Description
tests/​unit/​models/​test_seed_group.py	Adds regression coverage for nested metadata isolation.
pyrit/​models/​seeds/​seed_group.py	Deep-copies metadata when constructing message pieces.

_{Review effort: Balanced}