test(axum-extra): improve response_not_set_filename test invariants

[darth-raijin]

Strengthen test invariants for file_stream.rs by deriving values where possible and replacing magic values with constants.

Motivation

To improve test quality and reduce brittleness when changing test data, this PR updates the response_not_set_filename test to derive values (like content-length) from the test payload and to use existing constants (like mime types and headers).

Solution

• Define file_content and size once at the top of the test to enable reuse for assertions and usage in the /file handler
• Replace hard coded header names, with constants from http::header
• Replace hard coded MIME string with constant from mime