[tests] handler decomposition

[blast-hardcheese]

Why

Striking a balance between test handler reuse and colocating the handler specifiers with the tests that use them.

What changed

• Decomposing monolithic server handler specifiers into directly DI-ing reusable components
• For tests that need bespoke handlers, defining those alongside the methods that use them.

Test plan

CI

[blast-hardcheese]

@cbrewster Following up on an earlier PR comment. This brings handler invocation closer to the tests that use them.

[cbrewster]

Nice, I like it