feat(contract): add AUTH_MATRIX.md compliance test suite

Adds tests/auth_matrix.rs to the subscription crate (which already
hosts cross-contract integration tests) covering all five contracts
listed in AUTH_MATRIX.md:

- myfans-token: initialize, set_admin, set_metadata, read views,
  approve, transfer_from, clear_allowance, mint, burn, transfer
- creator-registry: initialize, register_creator (admin + self),
  unregister_creator, get_creator_id
- subscription: init, create_plan, subscribe, read views,
  extend_subscription, cancel, pause/unpause, set_fee_recipient,
  set_fee_bps
- content-access: initialize, unlock_content, read views,
  set_content_price, set_max_price, set_admin
- earnings: init, record, get_earnings, withdraw

Each method has a valid invocation test (correct signer succeeds)
and an invalid invocation test (env.set_auths(&[]) strips auth so
require_auth() fails).

Cargo.toml updated to add creator-registry and earnings as
dev-dependencies and register the new [[test]] target.

Author: Inkman007

contract/contracts/subscription/Cargo.toml

@@ -16,6 +16,11 @@ name = "contract_integration"
 path = "tests/contract_integration.rs"
 required-features = ["testutils"]
 
+[[test]]
+name = "auth_matrix"
+path = "tests/auth_matrix.rs"
+required-features = ["testutils"]
+
 [features]
 testutils = ["soroban-sdk/testutils"]
 
@@ -27,4 +32,6 @@ myfans-lib = { path = "../myfans-lib" }
 soroban-sdk = { workspace = true, features = ["testutils"] }
 myfans_token = { package = "myfans-token", path = "../myfans-token" }
 content_access = { package = "content-access", path = "../content-access" }
+creator_registry = { package = "creator-registry", path = "../creator-registry" }
+earnings = { package = "earnings", path = "../earnings" }
 myfans_lib = { package = "myfans-lib", path = "../myfans-lib", features = ["testutils"] }