rpc : reject invalid top-level graph nodes in graph_compute

[liminfei-amd]

Overview

rpc_server::graph_compute() deserializes each top-level graph node id and
resolves it through create_node(). The old check only rejected a nullptr
result when the wire id was non-zero, treating a nullptr result for id 0
as "expected" and letting it into graph->nodes. id 0 is the wire sentinel
for "no tensor" and is valid for optional tensor sources (src[]/view_src),
but it is not a valid top-level graph node. The backend later dereferences
that node during graph planning, crashing the RPC server on a single crafted
GRAPH_COMPUTE message (device=0, n_nodes=1, node_id=0, n_tensors=0).

Fixes #25299.

Reject every failed top-level node reconstruction before calling
ggml_backend_graph_compute(), regardless of id.

Additional information

Verified with a localhost-only RPC client sending the malformed message above
against a CPU-only build (no ROCm/HIP): before the fix the server exits with
SIGSEGV; after the fix the malformed connection is closed and a subsequent
client can still connect and complete HELLO normally.

A regression test (test-rpc-invalid-graph-node) is included; it starts the
RPC server, sends the crafted message, confirms the connection is dropped, and
verifies the server stays alive for new connections.

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: YES - an AI assistant helped locate the root cause and drafted this fix; I reviewed the surrounding create_node/graph_compute code, reproduced the crash and the fix's effect with a local RPC client, and can explain every line of the change.

[liminfei-amd]

@ruixiang63 you reopened this PR back in July — would you be able to take a look,
or point it at whoever should review it?

Rebased onto current master. The branch had gone stale and was conflicting. The
only conflict was in tools/rpc/CMakeLists.txt, which gained the
test-rpc-multi-server block in #26500. Both tests are kept. Mine is now gated
on LLAMA_BUILD_TESTS — it was on the generic BUILD_TESTING before, which this
project does not use, so it would not have registered in a normal test build.

I also removed a branch that my own change had made dead: once the guard returns
on nullptr, the following if (graph->nodes[i] != nullptr) is always true.
Removing it is safe because create_node() returns non-null only after
tensor_ptrs.find(id) succeeds, so the tensor_ptrs.at(id) below it cannot
throw.

The crash still reproduces on current master. Same test harness on both sides,
with only the guard change differing:

	master	this PR
test-rpc-invalid-graph-node	FAILED	passed
server after a malformed GRAPH_COMPUTE	dies, SIGSEGV	logs the rejection and keeps serving

test-rpc-multi-server also passes on this branch, which covers the normal graph
path through the code I touched. Both builds are CPU-only.

One thing I cannot do from here: the workflow runs on this head are all
action_required, so CI has not started. If you are able to approve them, that
would help.