[FEAT] Add unit and integration tets and enable testability for core components

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### Requirements

- [x] I have searched the existing issues to ensure this feature has not already been requested.
- [x] I have clearly described the problem and the proposed solution.
- [x] I have considered alternative solutions.

### 1. Is your feature request related to a problem? Please describe.

I noticed that it currently lacks unit tests, which makes it a bit challenging to confidently extend, refactor, or verify the behavior of core components.

### 2. Describe the Solution You'd Like

It would be wonderful to have some foundational unit tests in place for key components, even if minimal, to establish a baseline. The main goal is to test critical components and make the architecture more testable if necessary. I am happy to help by submitting initial tests for review when I have time :)

### 3. Describe Alternatives You've Considered

I considered writing only integration tests by running a sample app and observing behavior, but these tests are slower and don't easily catch small logic bugs or regressions.

### 4. Proposed API (if applicable)

No new public API needed. Suggest adding `SoundFlow.UnitTests` and `SoundFlow.IntegrationTests` projects. Minor internal refactoring to support DI and mocking as needed.

### 5. Benefits

Improves code quality, eases contributions, and reduces bugs. It also might helps ensure robust handling of edge cases in audio processing.

### 6. Potential Drawbacks/Challenges

- Some effort may be needed to make parts of the code more testable.
- Testing audio processing can be complex and may require mocks or utilities for verifying stream content.
- Maintaining tests does introduce some overhead, though the long-term benefits usually outweigh this.

### 7. Additional Context

Projects like [NAudio](https://github.com/naudio/NAudio/tree/master/NAudioTests) offer some inspiration in terms of structure and testability. I’d love to see SoundFlow take a similar path, even with a modest starting point for tests.

Start by reviewing the repository structure and identifying the core components that lack coverage. Define the proposed SoundFlow.UnitTests and SoundFlow.IntegrationTests projects, then establish a small baseline of unit and integration tests for critical audio-processing behavior. Done means the projects are runnable, core behavior has regression coverage, and any required testability changes are documented and reviewable.

Written by the indexing model from the issue text.