Add unit tests for the `Duration` class

### 🆕🐥 Newcomer Friendly

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### 🐞 Problem Description

`Duration` is a small public value object (exported from `hiero_sdk_python/__init__.py`) that wraps a
number of seconds and converts to and from the protobuf `Duration` message. It is used wherever the
SDK needs a time span for example the auto-renew period on topics and accounts.

There is no `tests/unit/duration_test.py`. Nothing currently verifies that:

- passing a non-integer to `Duration(...)` raises the `TypeError` its `__post_init__` promises,
- `_to_proto()` / `_from_proto()` round-trip correctly,
- `__str__`, `__repr__` and `__eq__` behave as written (`__eq__` returns `False` — not
`NotImplemented` — for non-`Duration` objects, which is worth pinning down in a test).
Files to look at:

- `src/hiero_sdk_python/Duration.py` the code under test (37 lines; note the capital-D filename)
- `tests/unit/timestamp_test.py` an existing test for the closest sibling type; good reference for
how proto conversion is tested here
- `tests/unit/supply_type_test.py` reference for file layout and style

### 💡 Solution

Add `tests/unit/duration_test.py` covering construction, validation, protobuf conversion in both
directions, and the dunder methods. Tests only — no production code changes.

Please write the tests to describe the code's **current** behaviour. If you spot something that looks
questionable, don't change it: leave a comment on the issue and we'll open a separate issue for it.

### 🛠️ Implementation Steps

### 🧠 Good First Issue Developers — Prerequisites & Expectations

> [!IMPORTANT]
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>
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> [!NOTE]
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> 🧩 **Difficulty:** Small, well-contained change
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- [ ] **Windows users:** see the [Windows Setup Guide](https://github.com/hiero-ledger/hiero-sdk-python/blob/main/docs/sdk_developers/setup_windows.md) for platform-specific installation steps.

- [ ] **Sync Main** pull any recent upstream changes: [Guide](https://github.com/hiero-ledger/sdk-collaboration-hub/blob/main/guides/issue-progression/for-developers/contributor-setup.md#7-sync-main)

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You are set up! 🎉

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#### 🛠️ Solve the Issue

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- [ ] **Submit the pull request:** click `**Create pull request**` and `Ready to Review` 🎉

### ✅ PR Quality Checklist

To be able to close this issue, the following criteria must be met:

Workflow:

- [ ] I have worked from a branch that is up to date with main
- [ ] I have not made changes outside the scope of this issue
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- [ ] I linked the issue in my PR description using `Fixes #issue_number` to automatically close it when merged
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- [ ] I have installed and run **pre-commit** hooks to ensure code quality and formatting.

Issue Requirement:
- [ ] The issue is solved: I've carefully read and implemented the issue requirements
---

### 🤔 What to expect after submitting a PR

Once you open a pull request, here's what happens next.

**🤖 1. Automated checks**
Automated checks will run and all must pass before merging.
Open any failed check at the bottom of the PR to see details. Ask for help if you don't understand a failure or how to fix it.

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Once tests pass, a team member will check if your PR follows the workflow.

**✅ 3. Team review**
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You may be asked to make changes or your PR may be approved.
Approved PRs are usually merged within **one day**.

**🔄 Merge conflicts (sometimes)**
Conflicts can happen and are normal as the SDK updates. Resolve using this [Guide](https://github.com/hiero-ledger/sdk-collaboration-hub/blob/main/guides/issue-progression/for-developers/merge_conflicts.md)

> [!TIP]
> **Follow the workflow and double check your work**
> This is the best way to ensure a fast review and merge process.
---

### 🧭 Getting help if you're stuck

**🆘 Stuck?**

> [!TIP]
>
> - Comment on this issue and ask maintainers or tag `@good_first_issue_support_team`
>

#### Other Resources:

- [New Starter Docs (Signing, Rebasing, Changelog, Merge Conflicts, Workflow, Setup)](https://github.com/hiero-ledger/sdk-collaboration-hub/tree/main/guides/issue-progression/for-developers)
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