fix(pxe): return no shared secrets when scope keys are not held

[nchamo]

Problem

Simulating a utility function for a scope the PXE does not control (e.g. balance_of_private(A) on a PXE that only knows A as a registered sender) fails with "No public key registered" at Token.sync_state. The e2e tests have always expected this to read 0, and they used to pass, but only because the failing path was unreachable: with no handshakes on chain, sync skipped secret derivation entirely. Once onchain delivery started establishing a non-interactive handshake for every recipient (including each account's own constructor note), handshake discovery began deriving secrets for foreign scopes and hit the throw. The affected tests were skipped under TODO(F-741) until we could tackle this. This PR makes the path handle the no-keys case and restores the tests.

Fix

• getSharedSecrets returns an empty array instead of throwing when the scope's keys are not held. Controlled accounts always have their complete address registered, so an empty response uniquely means "no key access".
• get_shared_secrets accepts the empty response, so handshake discovery yields no secrets for a foreign scope.
• get_shared_secret returns Option<Field>, and message decryption treats none as undecryptable and skips the message. This path was also broken for foreign scopes, previously masked by the discovery throw.
• The two skipped e2e tests are unskipped. Standard contracts are re-pinned since aztec-nr changed.

Fixes F-741

[mverzilli]

Nice!