server: tests: fetch random media marker via /apply-template (#21962) (#21980)

server: tests: fetch random media marker via /apply-template (#21962) (#21980)

* server: tests: fetch random media marker via /apply-template (#21962 fix)

* server: allow pinning media marker via LLAMA_MEDIA_MARKER env var

get_media_marker() checks LLAMA_MEDIA_MARKER at first call and uses it
as-is if set, falling back to the random marker otherwise.

Tests no longer need to fetch the marker dynamically via /apply-template:
the fixture sets LLAMA_MEDIA_MARKER=<__media__> so the hardcoded prompts
work as before.

Address review feedback from ngxson

* server: make get_media_marker() thread-safe via magic statics

Use a C++11 static local with a lambda initializer instead of a global
static with an empty-check. The runtime guarantees initialization exactly
once without explicit locking.

Address review feedback from ggerganov

* nits

* nits