feat: introduce sandbox environment support

[paolodamico]

Fairly explanatory. Reference project linked in the code.

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Note

Medium Risk
New public enum variant affects all clients via UniFFI; misconfigured sandbox could point at wrong contracts or attestation trust if other env matches were missed (this diff only updates the sites shown).

Overview
Adds a third BedrockEnvironment::Sandbox value (UniFFI-exposed) with wire string "sandbox" and docs describing it as an external TFH testing environment that still uses World ID staging protocol settings.

Sandbox is treated like staging everywhere this PR touches environment branching: Nitro enclave attestation uses staging PCR configs and the staging AWS Nitro root cert; world_gift_manager_address and test helper address_book_address resolve to the same contract addresses as staging.

Minor comment whitespace fixes in nitro_enclave/mod.rs only.

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[brian-terczynski]

LGTM but will let someone with more familiarity provide the approval. Thanks for the changes!

[brian-terczynski]

Bumping this to approval so we can move forward.