feat(contracts): implement upgradeable smart contract pattern (closes #838)

[Blaqkenny]

Summary

Implements the on-chain upgrade pattern required by Stellar Wave issue #838 on `inheritance-contract`:

• Admin-gated `upgrade` entrypoint that swaps the deployed WASM hash via `env.deployer().update_current_contract_wasm`.
• Strict admin credentials: governance mutations require `admin.require_auth()` against the address stored under `InstanceDataKey::Admin` in instance storage.
• Persistent storage retention: the host wasm swap leaves `DataKey::Plan` storage, claim status, and the instance admin record untouched. Instance TTL is bumped inside `upgrade`, `initialize`, `set_admin`, and `version` so the admin record cannot expire across a swap.
• Storage layout additions:
  • New exports: `initialize`, `set_admin`, `get_admin`, `upgrade`, `version`.
  • New `Error` variants: `AlreadyInitialized` (9) and `NotInitialized` (10).
  • `CONTRACT_VERSION` is now `pub const` so off-chain clients can pin against it.

Acceptance criteria mapping

Issue #838 requirement	Implementation
`upgrade` entrypoint	`pub fn upgrade(env: Env, new_wasm_hash: BytesN<32>)` in `InheritanceContract`
WASM swap via Soroban host functions	`env.deployer().update_current_contract_wasm(new_wasm_hash)`
Admin credentials protection	Stored admin loaded via `authorize_admin` helper which calls `admin.require_auth()` before any state change
Persistent storage retention	Storage lives under `contract_id` keys; `update_current_contract_wasm` does not modify storage. Tests assert via `env.as_contract` reads
Tested upgrade scenarios	`test_upgrade_uninitialized_fails`, `test_upgrade_unauthorized_panics`, `test_upgrade_succeeds_when_admin_authorizes`, `test_upgrade_preserves_persistent_storage`, `test_upgrade_preserves_admin_in_instance_storage`

Tests

• 24 `cargo test -p inheritance-contract` cases now pass (was 13). The 11 new tests cover init success / duplicate-init guard / admin rotation / unauthorized `set_admin` / unauthorized `upgrade` / successful `upgrade` / persistent plan retention across `upgrade` / admin retention across `upgrade`.
• A real pre-built wasm fixture (`contracts/inheritance-contract/testdata/inheritance_contract.wasm`) is included so the host wasm parser accepts the `upload_contract_wasm` test call. Regenerate with:
  `cargo build --target wasm32-unknown-unknown --release -p inheritance-contract`

Files

• `contracts/inheritance-contract/src/lib.rs` — contract logic
• `contracts/inheritance-contract/src/test.rs` — tests
• `contracts/inheritance-contract/testdata/inheritance_contract.wasm` — wasm fixture for upgrade tests

Closes #838

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@Blaqkenny Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

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[ONEONUORA]

@Blaqkenny
Pls
Run cargo fmt --all -- --check
and clippy