refactor(dgw): simplify aead error mapping in credential crypto

[Copilot]

aead::Error is a unit struct that does not implement std::error::Error, making .context() inapplicable. Simplifies the two AEAD error sites to drop the unused binding since the type carries no diagnostic information.

Changes:

• encrypt / decrypt: map_err(|e| anyhow::anyhow!("...: {e}")) → map_err(|_| anyhow::anyhow!("..."))
• .context() is already used where applicable (e.g., String::from_utf8 on the decrypted bytes)

// Before
cipher.encrypt(&nonce, plaintext.as_bytes())
    .map_err(|e| anyhow::anyhow!("encryption failed: {e}"))?;

// After
cipher.encrypt(&nonce, plaintext.as_bytes())
    .map_err(|_| anyhow::anyhow!("encryption failed"))?;

Issue: #1689

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