Add unit tests for FilterChainImpl

[codebyNorthsteep]

Issue Description

Implement unit tests for FilterChainImpl to verify the core request-processing logic of the server.

Goal:

• Ensure that the server correctly manages the transition between security filters and the final request handling (TerminalHandler).

• Test Cases to Implement:

• Sequential Execution: Verify that filters are called in the order they are added and that the TerminalHandler is called last.

• Chain Interruption: Verify that if a filter (e.g., an IP-blocker) does not go on to the next filter (by not calling chain.doFilter()), the TerminalHandler is never reached.

Why this is needed:

• This is critical for our server's security. Without these tests, we cannot guarantee that our Filters actually have the power to block unauthorized requests before they reach our files.

[coderabbitai]

🔗 Related PRs

#46 - 23 define and create filter interface [merged]
#56 - Implement configurable filter pipeline (global + per‑route filters) #34 [merged]
#70 - Feature/27 ipfilter [merged]

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