Fetch switch port lldp neighbors API incorrectly returns 500 error when no record is found

The issue is seen on both rack2 and rack3 (the latter running on r14). Here is an example of the error:

$ RACK_ID=$(oxide api /v1/system/hardware/racks | jq -r .items[0].id)
$ echo $RACK_ID 
de608e01-b8e4-4d93-b972-a7dbed36dd22

$ oxide api /v1/system/hardware/rack-switch-port/$RACK_ID/switch0/qsfp0/lldp/neighbors
error; status code: 500 Internal Server Error
{
  "error_code": "Internal",
  "message": "Internal Server Error",
  "request_id": "ca53c840-b2cf-4d9c-90e2-da47057e7ae8"
}
error

Nexus log shows that it's actually a 404 error:

04:25:28.047Z INFO 65a11c18-7f59-41ac-b9e7-680627f996e7 (dropshot_external): request completed
    error_message_external = Internal Server Error
    error_message_internal = failed to get neighbor list for switch0/qsfp0: Error Response: status: 404 Not Found; headers: {"content-type": "application/json", "x-request-id": "bb315203-4219-478e-805c-24b4ec30bd41", "content-length": "130", "date": "Tue, 03 Jun 2025 04:25:28 GMT"}; value: Error { error_code: Some("no such interface: qsfp0/0"), message: "Not Found", request_id: "bb315203-4219-478e-805c-24b4ec30bd41" }
    file = /home/build/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/dropshot-0.16.2/src/server.rs:855
    latency_us = 316829
    local_addr = 172.30.2.5:443
    method = GET
    remote_addr = 172.20.17.42:51159
    req_id = ca53c840-b2cf-4d9c-90e2-da47057e7ae8
    response_code = 500
    uri = /v1/system/hardware/rack-switch-port/de608e01-b8e4-4d93-b972-a7dbed36dd22/switch0/qsfp0/lldp/neighbors

Start by reproducing the provided oxide API request for the rack switch-port LLDP neighbors endpoint and compare its response with the Nexus log. Trace how the upstream 404 for a missing interface is handled; done means this condition is exposed as a 404 response instead of being converted to a 500 Internal Server Error.