fix: inject the routing-observations reader instead of mocking the proxy module

The handler imported getRoutingObservations directly, so its test had to
replace the whole "@better-ccflare/proxy" module via mock.module. Bun
applies module mocks PROCESS-WIDE across file boundaries, so that mock
also replaced the module the proxy package's own tests import: six of
their assertions read through the mock's empty record instead of the
real singleton they had just written to.

The failure appears only when both test files run in one `bun test`
invocation (a whole-repo run, or any invocation naming both paths) and
is order-independent, so the package-scoped runs used during development
stayed green and hid it.

The handler now takes its reader as a parameter defaulting to the real
getRoutingObservations, and the test passes fakes directly — no module
mocking, no cross-suite coupling. Verified by running both files
together in both orders: 12 pass, 0 fail (was 6 fail). Production
behaviour is unchanged; callers use the default.