feat: Add arithmetic operators to Hbar (+, -, abs)

### 🧑‍🎓 Beginner Issue

This issue adds arithmetic operators (`+`, `-`, `abs`) to the `Hbar` class, bringing it in line with Python conventions and making it easier to work with HBAR amounts in user code.

It involves editing one file (`hbar.py`), adding three short methods, and writing unit tests. No network calls, no protobuf, no breaking changes — a great first contribution to get familiar with the codebase.

### 🐞 Problem Description

The `Hbar` class in `src/hiero_sdk_python/hbar.py` supports comparison operators
(`<`, `>`, `<=`, `>=`, `==`) but has no arithmetic operators. This means developers
cannot add, subtract, or get the absolute value of two `Hbar` values using Python's
native syntax.

Current behaviour:
```python
a = Hbar(10)
b = Hbar(5)
a + b # TypeError: unsupported operand type(s) for +: 'Hbar' and 'Hbar'
a - b # TypeError
abs(a) # TypeError

### 💡 Expected Solution

****Proposed** Change**
Add the following dunder methods to hbar.py:

- __add__(self, other: Hbar) -> Hbar

- __sub__(self, other: Hbar) -> Hbar

- __abs__(self) -> Hbar

All three should operate on the internal _amount_in_tinybar value and return a new Hbar instance via Hbar.from_tinybars(...). Non-Hbar operands should return NotImplemented.

Unit tests should be added to hbar_test.py .

### 🔍 Research Pointers

- Main file to edit:
hbar.py

- Existing tests to understand the class:
hbar_test.py

- The class already has __lt__, __le__, __gt__, __ge__ — follow the same pattern for the new methods

- The existing negated() method and from_tinybars() classmethod show how to construct a new Hbar from a tinybar value — you'll use the same approach

- Python docs on arithmetic dunder methods: https://docs.python.org/3/reference/datamodel.html#emulating-numeric-types

- For reference, the Hiero JS SDK exposes .plus() and .minus() on its Hbar type

### 🛠️ Implementation Notes

- All three methods should operate on the internal _amount_in_tinybar integer value

- Return a new Hbar instance using Hbar.from_tinybars(result)

- If the operand is not an Hbar instance, return NotImplemented (same pattern as the existing comparison operators)

- __abs__ takes no second argument — it just returns Hbar.from_tinybars(abs(self._amount_in_tinybar))

- The existing negated() method should remain unchanged for backwards compatibility

### 🧠 Beginner Contributors — Prerequisites & Expectations

- You should be comfortable reading and writing Python classes

- You should understand what dunder (magic) methods are in Python

- You should have the repo set up locally and be able to run the unit tests (uv run pytest tests/unit/hbar_test.py)

- Please comment "I'd like to work on this" before starting — one contributor will be assigned at a time

### 🧪 Testing Requirements

Add tests to hbar_test.py covering:
- Hbar(10) + Hbar(5) equals Hbar(15)

- Hbar(10) - Hbar(5) equals Hbar(5)

- abs(Hbar(-3)) equals Hbar(3) and abs(Hbar(3)) equals Hbar(3)

- Adding a non-Hbar type raises TypeError (e.g. Hbar(1) + 5)

- Subtracting a non-Hbar type raises TypeError

- Edge cases: adding Hbar.ZERO, subtracting to get a negative result

Run tests with:
```python
uv run pytest tests/unit/hbar_test.py -v
```

### 🛡️ Quality & Review Standards

- Follow the existing code style in hbar.py (type hints, docstrings, return types)

- No new dependencies

- No breaking changes to existing behaviour

- Ruff linting must pass: uv run ruff check src/hiero_sdk_python/hbar.py

### ✅ PR Quality Checklist

- [ ] __add__, __sub__, and __abs__ are implemented in hbar.py

- [ ] All new methods have type hints and docstrings

- [ ] Unit tests added and passing

- [ ] uv run ruff check passes with no errors

- [ ] PR title follows conventional commits format: feat: add arithmetic operators to Hbar

- [ ] Commits are GPG and DCO signed

### 📋 Workflow quick reference

You have done this before — here are the links if you need them:

| Step | Guide |
|------|-------|
| Claim this issue | Comment `/assign` below |
| Sync with main | [Rebasing guide](https://github.com/hiero-ledger/sdk-collaboration-hub/blob/main/guides/issue-progression/for-developers/rebasing.md) |
| Open a PR and link this issue | [PR guide](https://github.com/hiero-ledger/sdk-collaboration-hub/blob/main/guides/issue-progression/for-developers/contributor-workflow.md) |
| Resolve merge conflicts | [Merge conflicts guide](https://github.com/hiero-ledger/sdk-collaboration-hub/blob/main/guides/issue-progression/for-developers/merge_conflicts.md) |

### 📚 Resources & Support

**🆘 Stuck?**
> [!TIP]
> **Comment on this issue:** and describe what you have tried. A maintainer will respond.

**Python SDK References:**
- [Windows Setup Guide](https://github.com/hiero-ledger/hiero-sdk-python/blob/main/docs/sdk_developers/setup_windows.md) — platform-specific installation steps
- [Pylance guide](https://github.com/hiero-ledger/hiero-sdk-python/blob/main/docs/sdk_developers/pylance.md) — inline type checking during development
- [Running examples](https://github.com/hiero-ledger/hiero-sdk-python/blob/main/docs/sdk_developers/examples.md)
- [Browse closed beginner PRs](https://github.com/hiero-ledger/hiero-sdk-python/pulls?q=is%3Apr+is%3Amerged+label%3A%22skill%3A+beginner%22) — see how others did it

**References:**
- [Hedera Protobufs](https://github.com/hashgraph/hedera-protobufs)
- [Community Calls](https://zoom-lfx.platform.linuxfoundation.org/meetings/hiero?view=week)
- [Discord](https://github.com/hiero-ledger/sdk-collaboration-hub/blob/main/guides/issue-progression/for-developers/discord.md)