fix(rpc): Improve input validation and error handling (llama/13069)

fix(rpc): Improve input validation and error handling (llama/13069)

* fix(rpc): Improve input validation and error handling

The `rpc-server` was vulnerable to Denial of Service attacks via
several RPC commands (`SET_TENSOR`, `GRAPH_COMPUTE`, etc.). Malformed
messages could trigger failed assertions (e.g., invalid `ggml_type`)
or out-of-bounds reads/writes leading to `GGML_ABORT` calls,
crashing the server process.

This PR introduces robust input validation and replaces `abort()`
calls with graceful error handling:

- **Type Validation:** `deserialize_tensor` now checks if the
  `tensor->type` is within the valid `GGML_TYPE_COUNT` range
  *before* calling `ggml_new_tensor_4d`. Returns `nullptr` on
  invalid type.
- **Bounds Checks:** Replaced `GGML_ABORT` in `set_tensor`,
  `set_tensor_hash`, and `get_tensor` handlers with error
  logging and returning `false` when data/offset parameters
  are out of buffer bounds.
- **Size Checks:** Added safe arithmetic checks (for overflow) in
  `graph_compute` when calculating required message sizes based
  on client-provided `n_nodes` and `n_tensors`. Returns early
  if the reported sizes conflict with the actual message size or
  would lead to overflow.
- **Error Propagation:**
    - `create_node` now checks for `nullptr` return values from
      `deserialize_tensor` and its recursive calls, propagating
      `nullptr` upwards on failure. Uses `find` instead of `at`
      for safer map access.
    - `copy_tensor` now checks for `nullptr` from `deserialize_tensor`
      and sets the response status to failure if deserialization
      or bounds checks fail.
    - `graph_compute` now checks for `nullptr` return from
      `create_node` and returns failure status correctly. The final
      return value now reflects the actual computation status.

These changes improve the RPC server's resilience
against malformed client requests, preventing crashes and ensuring
errors are handled more gracefully.

Signed-off-by: Ville Vesilehto <redacted>
* refactor(rpc): address pr comments

removed comments and unnecessary returns

Signed-off-by: Ville Vesilehto <redacted>
* refactor(rpc): ambiguous nullptr from create_node

rpc_server::create_node could previously return nullptr if the input ID
was 0 (valid) or if an internal error (deserialization, recursion
failure) occurred (invalid). This ambiguity made error handling
difficult for the caller (`graph_compute`).

This commit clarifies the meaning of nullptr:
- `graph_compute` now checks if the input 'id' was non-zero when
  `create_node` returns nullptr, correctly identifying failures
  versus intentional null links.
- `create_node` avoids recursive calls for zero IDs and propagates
  nullptr unambiguously on failure during recursion.

Signed-off-by: Ville Vesilehto <redacted>
* refactor(rpc): initial zero check in create_node

The caller (`graph_compute`) already checks `id != 0` when handling
a `nullptr` return from `create_node`, correctly distinguishing
intentional null links from actual errors. This makes the initial
`if (id == 0)` check redundant.

Also removes the log message when a tensor ID is not found in the
provided map which was added in this branch.

Signed-off-by: Ville Vesilehto <redacted>
* fix(rpc): Handle get_alloc_size failure in server

Check the return value of `server.get_alloc_size` in the RPC server
loop. If the call fails, return early to close the connection.

Signed-off-by: Ville Vesilehto <redacted>
* refactor(rpc): input size validation in graph_compute

Removes detailed, step-by-step size calculations and overflow
checks in favor of simpler direct comparisons, assuming 64-bit
overflow is unlikely.

Signed-off-by: Ville Vesilehto <redacted>
* refactor(rpc): remove extra status code setting

Removes the explicit setting of `response.result = GGML_STATUS_FAILED`
when `create_node` returns `nullptr` within `graph_compute`.
Primary signal is the `false` return value in case of failure.

Signed-off-by: Ville Vesilehto <redacted>
* refactor(rpc): remove redundant check for tensor->type

Breaks CI on ubuntu-cpu-make. Tensor type is uint32_t, thus
the check is not needed.

Signed-off-by: Ville Vesilehto <redacted>
---------

Signed-off-by: Ville Vesilehto <redacted>