test(example): pin the buyer gate on register_with

[ogundeleoluwaferanmi35]

Closes #12

Summary

The existing register_with_needs_the_buyers_authorization test removes all
authorizations (env.set_auths(&[])) and asserts the call fails. That only proves some
authorization is required, not that it is the buyer's — it would pass unchanged if the gate
were config.seller.require_auth() or any other address in EscrowConfig. Since the buyer gate is
what stops an arbitrary caller from publishing a manifest under the escrow's address, this PR pins
it with tests that exercise specific addresses.

What changed

Two new tests in examples/long_escrow/src/test.rs, using the SDK's mock_auths (which replaces
the fixture's blanket mock_all_auths with a specific list of authorized addresses):

• register_with_rejects_non_buyer_authorizations — authorizes the seller and a wholly
  unrelated generated address, but not the buyer, and asserts register_with still fails and
  never reaches the registry.
• register_with_succeeds_with_only_the_buyers_authorization — authorizes only the buyer and
  asserts register_with succeeds and the registry records the escrow.

The existing set_auths(&[]) test is untouched and still covers the no-auth case.

Key design decisions

• Used mock_auths rather than hand-built set_auths XDR entries: it registers the no-op
  MockAuthContract at each address, which is how Void-signature credentials are accepted in
  enforcing mode. It also disables the mock_all_auths from the fixture, so a success result is
  genuinely attributable to the authorized address alone.
• A small register_with_invocation helper builds the mocked MockAuthInvoke. buyer.require_auth()
  (no args) authorizes the current frame, so the entry must name the escrow, the function, and
  exactly the arguments the call passes — using the same .into_val() conversions the generated
  client uses, so the frames compare equal.
• The registry cross-call succeeds in the buyer-only test because the fake registry's
  contract.require_auth() is satisfied by the invoker check (the escrow is the caller), not by a
  second auth entry.

Acceptance criteria

• A test authorizes the seller + a wholly unrelated address and asserts register_with still fails — register_with_rejects_non_buyer_authorizations
• A test authorizes the buyer specifically (not all auths) and asserts success — register_with_succeeds_with_only_the_buyers_authorization
• Changing the gate to any other EscrowConfig address makes at least one test fail — verified locally by temporarily setting config.seller.require_auth(): both new tests failed; reverted before submitting
• The existing set_auths(&[]) test stays

Checks

All four CI checks pass locally:

cargo fmt --all -- --check          # clean
cargo clippy --all-targets -- -D warnings   # clean
cargo test --all                    # 26 passed (7 in long_escrow, incl. 2 new)
stellar contract build              # build complete

Follow-ups

None. The change is limited to test-only code in examples/long_escrow.

Security note

Test-only change; no production logic touched. No secrets or environment variables involved.