fix(backends): preserve an explicit seed of 0 in sglang and vllm

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#11772 exempted Temperature from the zero-filter in both backend adapters,
because proto3 has no field presence and an explicit 0 is indistinguishable
from "unset". Seed has exactly the same property and is still filtered:

    if proto_field != "Temperature" and value in (None, 0, 0.0, [], False, ""):
        continue

A caller pinning `"seed": 0` for a reproducible run therefore gets a random
seed instead, with no error and no log line — the one case where the failure
is invisible precisely because the request looked deliberate.

Both adapters now share a named tuple of fields whose zero is meaningful, so
the next one is added in one place rather than as a second special case.

Deliberately left filtered: top_k, top_p, min_p and the penalties. Their zero
is not a value a caller means — sglang disables top_k with -1, not 0, so
forwarding 0 there would turn a default into an invalid argument.

Verified on the sglang backend (Qwen3.5-MoE, arm64): with the temperature fix
alone, two identical requests at temperature 0 are byte-identical, but pinning
seed 0 has no effect until this change.

Signed-off-by: pos-ei-don <1822533+pos-ei-don@users.noreply.github.com>

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