[Beginner]: Add `operator==` to `PendingAirdropRecord`

### 🐥 Beginner Friendly

This issue is a great fit for contributors who are ready to explore the Hiero C++ codebase a little more and take on slightly more independent work.

Beginner Issues often involve reading existing C++ code, understanding how different parts of the SDK fit together, and making small, thoughtful updates that follow established patterns.

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> [!IMPORTANT]
> ### 🐥 About Beginner Issues
>
> Beginner Issues are a great next step for contributors who feel comfortable with the basic project workflow and want to explore the codebase a little more.
>
> These issues often involve:
> - Reading existing C++ code
> - Understanding how different parts of the SDK fit together
> - Making small, thoughtful updates that follow established patterns
>
> You'll usually see Beginner Issues focused on things like:
> - Small, well-scoped improvements to existing tests
> - Narrow updates to `src` functionality (e.g. refining helpers or improving readability)
> - Documentation or comment clarity
> - Enhancements to existing examples
>
> Other types of contributions — such as brand-new features, broader system changes, or deeper technical work — are just as valuable and may use different labels.

### 👾 Description of the Issue

The `PendingAirdropRecord` class is missing an `operator==` implementation.

Relevant files:
```
src/sdk/main/include/PendingAirdropRecord.h
src/sdk/main/src/PendingAirdropRecord.cc
```

`PendingAirdropId` (the type of one of its fields) already has `operator==` — see the recently merged companion PR. `PendingAirdropRecord` wraps a `PendingAirdropId` with an amount and should also support equality comparison.

### 💡 Proposed Solution

Add `operator==` to `PendingAirdropRecord` as a `const` member function:

- Declare `[[nodiscard]] bool operator==(const PendingAirdropRecord& rhs) const` in the header
- Implement it in `PendingAirdropRecord.cc`, comparing both member fields:
- `mPendingAirdropId`, `mAmount`
- Add or extend the unit test file in `src/sdk/tests/unit/PendingAirdropRecordTest.cc`

The change should not affect any existing behavior or public API.

### 👩‍💻 Implementation Steps

- [ ] Open `src/sdk/main/include/PendingAirdropRecord.h` and add the member `operator==` declaration inside the class body
- [ ] Open `src/sdk/main/src/PendingAirdropRecord.cc` and implement `operator==` comparing `mPendingAirdropId` and `mAmount`
- [ ] Open `src/sdk/tests/unit/PendingAirdropRecordTest.cc` and add equality tests:
- Two default-constructed instances are equal
- Two identically-constructed instances are equal
- Instances differing in each field are not equal
- [ ] Build the SDK and confirm all tests pass

### ✅ Acceptance Criteria

- [ ] **Scope:** Changes are limited to `PendingAirdropRecord.h`, `PendingAirdropRecord.cc`, and its unit test
- [ ] **Behavior:** No other SDK behavior or API changes
- [ ] **Tests:** Existing and new equality tests pass
- [ ] **Review:** All code review feedback addressed

---

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### 🤔 Additional Information

Declare `operator==` as a `const` member function — not a `friend`. The declaration in the header is `[[nodiscard]] bool operator==(const T& rhs) const;` and the definition in the `.cc` file uses `mField` directly (not `lhs.mField`) since `this` is the left-hand side.

If you have questions while working on this issue, feel free to ask! [Hiero-SDK-C++ Discord](https://discord.com/channels/905194001349627914/1337424839761465364)

Start with src/sdk/main/include/PendingAirdropRecord.h and the existing equality pattern for PendingAirdropId, then inspect src/sdk/main/src/PendingAirdropRecord.cc. Extend src/sdk/tests/unit/PendingAirdropRecordTest.cc with the listed equal and unequal cases, build the SDK, and confirm the existing and new tests pass.

Written by the indexing model from the issue text.