docs: reflect typed error sources across the workspace

[mintlify]

Summary

Aligns the docs with oxidezap/whatsapp-rust#597, which preserves typed error sources across the workspace instead of stringifying them.

Changes

• api/store.mdx — Updated the StoreError enum to the new shape (Connection, Database, Migration, Serialization now carry #[source] Box<dyn Error + Send + Sync>; new Validation, RetriesExhausted, InvalidConfig variants; removed NotFound, Backend). Added a note on is_database_busy_or_locked(). Updated the Redis example to wrap inner errors with Box::new(e) instead of e.to_string().
• guides/custom-backends.mdx — Updated StoreError::Serialization / StoreError::Database call sites to use Box::new(e). Replaced the now-removed StoreError::Deserialization with StoreError::Serialization.
• advanced/websocket-handling.mdx — Replaced SocketError::Crypto(String) and NoiseHandshake(String) with the new Cipher(NoiseError) and Marshal(BinaryError) variants. Updated the decrypt_frame example accordingly. Added a note on walking source() to downcast.
• concepts/architecture.mdx — Updated the SocketError snippet in the error-handling section.
• concepts/authentication.mdx — Replaced the PairCodeError::CryptoError(String) / RequestFailed(String) match with the typed split: InvalidPrimaryEphemeralKey, InvalidPrimaryIdentityKey, EphemeralKeyAgreement, IdentityKeyAgreement, AdvSecretKeyDerivation, BundleKeyDerivation, BundleAead. Wrapped everything in the new outer PairError::PairCode / PairError::RequestFailed(IqError) enum.
• api/client.mdx — Updated pair_with_code return type to Result<String, PairError> and refreshed the error table.

Validation

• mint validate — passes
• mint broken-links — no broken links

Source: oxidezap/whatsapp-rust@0b785d1