[Beginner]: Promote `toEvmAddress()` to a virtual method on `PublicKey` returning `std::optional<EvmAddress>`

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>
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>
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>
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### 👾 Description of the Task

`toEvmAddress()` is currently declared only on the concrete subclass `ECDSAsecp256k1PublicKey` ([src/sdk/main/include/ECDSAsecp256k1PublicKey.h:188](../../src/sdk/main/include/ECDSAsecp256k1PublicKey.h)):

```cpp
[[nodiscard]] EvmAddress toEvmAddress() const;
```

Any code holding a generic `std::shared_ptr` (which is what `PrivateKey::getPublicKey()` returns) must downcast before it can derive an EVM address. This pattern is scattered across the SDK and tests today:

```cpp
// src/tck/src/key/KeyService.cc:142-145
return std::dynamic_pointer_cast(privateKey->getPublicKey())
->toEvmAddress()
.toString();

// src/sdk/tests/integration/TransferTransactionIntegrationTests.cc:104-105
const EvmAddress evmAddress =
std::dynamic_pointer_cast(privateKey->getPublicKey())->toEvmAddress();
```

The sibling SDKs already expose this on the base public-key type: the JS SDK returns `undefined` and the Go SDK returns `""` for non-ECDSA keys. The C++ SDK should follow the same pattern with `std::optional`.

Relevant files:

```
src/sdk/main/include/PublicKey.h
src/sdk/main/src/PublicKey.cc
src/sdk/main/include/ECDSAsecp256k1PublicKey.h
src/sdk/main/src/ECDSAsecp256k1PublicKey.cc
src/tck/src/key/KeyService.cc
src/sdk/tests/unit/ECDSAsecp256k1PublicKeyUnitTests.cc
src/sdk/tests/unit/ED25519PublicKeyUnitTests.cc
src/sdk/tests/unit/AccountCreateTransactionUnitTests.cc
src/sdk/tests/integration/AccountCreateTransactionIntegrationTests.cc
src/sdk/tests/integration/TransferTransactionIntegrationTests.cc
```

### 💡 Proposed Solution

Add a virtual method to `PublicKey` with a default implementation returning `std::nullopt`, and override it in `ECDSAsecp256k1PublicKey` with the existing Keccak-256 derivation:

```cpp
// PublicKey.h (declare; define in PublicKey.cc to keep EvmAddress a forward declaration)
/**
* Get the EVM address derived from this PublicKey.
*
* @return The EvmAddress derived from this PublicKey, or std::nullopt if this key is not an
* ECDSAsecp256k1PublicKey.
*/
[[nodiscard]] virtual std::optional toEvmAddress() const;
```

```cpp
// ECDSAsecp256k1PublicKey.h — change the existing declaration to an override
[[nodiscard]] std::optional toEvmAddress() const override;
```

The body of the existing override in [src/sdk/main/src/ECDSAsecp256k1PublicKey.cc:404](../../src/sdk/main/src/ECDSAsecp256k1PublicKey.cc) stays exactly as it is — the `EvmAddress` it returns converts implicitly to `std::optional`.

**Compatibility note (please preserve this in the PR description):** adding the base-class virtual with a default implementation is backward compatible for code holding `PublicKey*`/`std::shared_ptr`. However, the return type of the existing `ECDSAsecp256k1PublicKey::toEvmAddress()` changes from `EvmAddress` to `std::optional` (C++ does not allow non-covariant return-type overloading), so call sites on the typed subclass need a trivial `.value()` / `->` adjustment. All in-repo call sites are enumerated below.

### 👩‍💻 Implementation Steps

- [ ] In [src/sdk/main/include/PublicKey.h](../../src/sdk/main/include/PublicKey.h), add `#include `, forward-declare `class EvmAddress;` next to the existing `class AccountId;` forward declaration, and declare the new virtual method shown above (a good spot is next to `toAccountId` around line 138).
- [ ] In [src/sdk/main/src/PublicKey.cc](../../src/sdk/main/src/PublicKey.cc), define the default implementation: `return std::nullopt;` (include `EvmAddress.h` there).
- [ ] In [src/sdk/main/include/ECDSAsecp256k1PublicKey.h](../../src/sdk/main/include/ECDSAsecp256k1PublicKey.h) (line 188) and [src/sdk/main/src/ECDSAsecp256k1PublicKey.cc](../../src/sdk/main/src/ECDSAsecp256k1PublicKey.cc) (line 404), change the return type to `std::optional` and mark the declaration `override`.
- [ ] Update the typed call sites (mechanical):
- `src/sdk/main/src/AccountCreateTransaction.cc:67` and `:88` — **no change needed**; `mAlias` is already a `std::optional`, so the assignment compiles as-is.
- `src/tck/src/key/KeyService.cc:143`, `:151`, `:160` — change `.toString()` on the result to `->toString()`. Optional cleanup: with the new virtual, the public-key branch no longer needs `std::dynamic_pointer_cast` at all — it can call `key->toEvmAddress()` directly and treat `std::nullopt` as the "not ECDSA" error case.
- `src/sdk/tests/unit/ECDSAsecp256k1PublicKeyUnitTests.cc:415` and `src/sdk/tests/unit/AccountCreateTransactionUnitTests.cc:144` — append `.value()`.
- `src/sdk/tests/integration/AccountCreateTransactionIntegrationTests.cc` lines 94, 241, 287, 338, 379, 432, 562, 652 and `src/sdk/tests/integration/TransferTransactionIntegrationTests.cc:105` — append `.value()`. (At lines 379, 432, and 105 the surrounding `std::dynamic_pointer_cast` may also be dropped, since the base class now exposes the method.)
- [ ] Add unit tests:
- In [src/sdk/tests/unit/ED25519PublicKeyUnitTests.cc](../../src/sdk/tests/unit/ED25519PublicKeyUnitTests.cc): calling `toEvmAddress()` on an `ED25519PublicKey` (through a `std::shared_ptr`) returns `std::nullopt`.
- In [src/sdk/tests/unit/ECDSAsecp256k1PublicKeyUnitTests.cc](../../src/sdk/tests/unit/ECDSAsecp256k1PublicKeyUnitTests.cc): calling `toEvmAddress()` through a base-class `std::shared_ptr` returns an engaged optional equal to the address returned by the typed subclass call.
- [ ] Build and run the affected unit suites:
```bash
cmake --preset linux-x64-debug -DBUILD_TESTS=ON
cmake --build --preset linux-x64-debug -j 6
ctest -C Debug --test-dir build/linux-x64-debug \
-R "ECDSAsecp256k1PublicKeyUnitTests|ED25519PublicKeyUnitTests|AccountCreateTransactionUnitTests"
```
(The integration-test edits are compile-only fixes; CI runs them against Solo.)
- [ ] Run `clang-format-17` against any file you touched.

### ✅ Acceptance Criteria

- [ ] **Scope:** Changes are limited to the files listed above.
- [ ] **Behavior:** `PublicKey::toEvmAddress()` returns `std::nullopt` for ED25519 keys and the Keccak-256-derived address for ECDSAsecp256k1 keys. No other behavior changes.
- [ ] **API:** The subclass return-type change and its migration (`.value()` / `->`) are called out in the PR description.
- [ ] **Tests:** New unit tests cover both the `std::nullopt` path (ED25519) and the engaged path (ECDSA) through a base-class pointer; all existing tests still pass.
- [ ] **Review:** All code review feedback addressed.

---

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### 🤔 Additional Information

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