test(zcash_address): Add unit tests for has_receiver_of_type, contains_receiver, and can_receive_memo

[Cosmos-Harry]

Summary

The three public query methods on unified::Address, has_receiver_of_type,
contains_receiver, and can_receive_memo, had no test coverage. This PR adds
a focused unit test covering their behaviour across meaningful cases.

What's tested

• has_receiver_of_type returns correct results for Orchard, Sapling, and
  Transparent pool types
• can_receive_memo returns true when shielded receivers are present, and
  false for transparent-only addresses
• contains_receiver requires exact byte-for-byte equality, not just type match
• A transparent-only address correctly reports no memo capability and no shielded
  pool support
• An Unknown receiver does not contribute to any pool type query — guarding
  against future regressions where a new receiver type might accidentally be
  mapped incorrectly

Why

These methods are part of the public API and are likely called by wallets to make
decisions about memo sending and receiver selection. A silent regression here
could cause real user-facing bugs. The Unknown receiver case in particular is
non-obvious from reading the code alone and benefits from being pinned by a test.

Notes

No behaviour changes. Test-only.