[PM-40646] Fix watch crash on corrupt encrypted data

[potatosips]

🎟️ Tracking

Fixes #2895

📔 Objective

Prevent the Bitwarden Watch app from terminating when locally persisted AES-GCM-encrypted Core Data data is malformed or can no longer be decrypted, for example after corruption or a key mismatch.

CryptoService now catches AES-GCM sealing and opening failures, logs them, and returns nil, allowing the Core Data transformer to treat unavailable data safely.

🧪 Validation

• git diff --check
• Source check confirms CryptoService no longer contains try! AES-GCM operations or plainData! unwraps.
• watchOS runtime testing is pending because this audit was performed on Windows without Xcode/watchOS tooling.

🔒 Security impact

No confidentiality or integrity bypass was demonstrated; this is an availability and robustness fix.

[CLAassistant]

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[bitwarden-bot]

Thank you for your contribution! We've added this to our internal tracking system for review.
ID: PM-40646
Link: https://bitwarden.atlassian.net/browse/PM-40646

Details on our contribution process can be found here: https://contributing.bitwarden.com/contributing/pull-requests/community-pr-process.