server : add request path to 404 error message

[Jinpop]

Overview

This PR updates the 404 response to include the requested path.

Previously, missing routes returned only "File Not Found", which made it unclear which path caused the error. This led to confusion in cases like #24224, where the user thought the server was running on the wrong port.

Fixes #24279

Additional information

Before:

{"error":{"message":"File Not Found","type":"not_found_error","code":404}}

After:

{"error":{"message":"File Not Found: /app","type":"not_found_error","code":404}}

Tests

Added test_404_error_includes_path, which verifies that a request to a missing route returns 404 and includes the requested path in the error message.

Local Verification

Verified locally on macOS Apple Silicon. The new test and the existing test_no_webui test both pass.

The path is also safe to include even when it contains invalid UTF-8, since the response is serialized through safe_json_to_str, which already uses error_handler_t::replace.

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: YES

The design decisions were made by me, including the response message format, keeping the existing "File Not Found" prefix for compatibility, and defining the test scenario.

AI was used to assist with issue and codebase exploration, build/reproduction/test execution, typing the code according to my design, and reviewing the changes. The commit message wording was also suggested by AI.

I personally reviewed and understood every changed line in this PR.