UDP check sometimes stays in "passing" status while endpoint is actually down

#### Overview of the Issue

On some machines we've experienced an issue with UDP healtcheck passing once and then hanging in "passing" status despite service going down and no longer being available. Monitoring the UDP traffic, consul sends just one check and stops sending any more. We're using a simple server that responds to an empty UDP payload with empty UDP payload.

Adding extra logging steps to consul, seems like the check just hangs on https://github.com/hashicorp/consul/blob/c1a887e0765ef8776a81569334db792174885755/agent/checks/check.go#L835

Looking through the code further, it seems like there's no actual timeout set for UDP checks. Dialer timeout set in `func (c *CheckUDP) Start()`, as far as I understand, is related to establishing connection (which makes no sense for UDP? except for timeout for resolving endpoint address?) and is irrelevant to actual `conn` timeouts, which should be set via `SetReadDeadline`? In `func (c *CheckUDP) check()`:
```go
deadline := time.Now().Add(c.Timeout)
err := conn.SetReadDeadline(deadline)
```

However just adding `SetReadDeadLine` doesn't fix anything because of check in https://github.com/hashicorp/consul/blob/c1a887e0765ef8776a81569334db792174885755/agent/checks/check.go#L836-L839

which I also do not understand at all. Why on read timeout, the check actually passess? Doesn't UDP healthcheck expect a response?

Setting `SetReadDeadLine` and deleting the "i/o timeout" case error check (btw why does it check for string value? I'm not a go developer, but this seems odd to me) makes the check update accordingly when the UDP endpoint goes up or down.

Since I'm not a go dev, I might be grossly misinterpreting what aformentioned functions do, but after making the changes I've described, consul stopped misreporting UDP service status for me.

---

#### Reproduction Steps

1. Start consul agent
2. Register servce with UDP healtcheck with timeout of 1s
3. Pass the check once, stop the service
4. Service is still passing in consul (logs, UI, API)

### Consul info for both Client and Server

Server info

```
agent:
check_monitors = 0
check_ttls = 0
checks = 0
services = 0
build:
prerelease = dev
revision = c1a887e0
version = 1.21.0
version_metadata =
consul:
acl = disabled
bootstrap = true
known_datacenters = 1
leader = true
leader_addr = 10.253.253.100:8300
server = true
raft:
applied_index = 565
commit_index = 565
fsm_pending = 0
last_contact = 0
last_log_index = 565
last_log_term = 2
last_snapshot_index = 0
last_snapshot_term = 0
latest_configuration = [{Suffrage:Voter ID:4ee65dbe-b13c-418f-c3b3-5698213eaea4 Address:10.253.253.100:8300}]
latest_configuration_index = 0
num_peers = 0
protocol_version = 3
protocol_version_max = 3
protocol_version_min = 0
snapshot_version_max = 1
snapshot_version_min = 0
state = Leader
term = 2
runtime:
arch = amd64
cpu_count = 5
goroutines = 190
max_procs = 5
os = linux
version = go1.23.3
serf_lan:
coordinate_resets = 0
encrypted = false
event_queue = 1
event_time = 2
failed = 0
health_score = 0
intent_queue = 1
left = 0
member_time = 2
members = 1
query_queue = 0
query_time = 1
serf_wan:
coordinate_resets = 0
encrypted = false
event_queue = 0
event_time = 1
failed = 0
health_score = 0
intent_queue = 0
left = 0
member_time = 1
members = 1
query_queue = 0
query_time = 1
```

```
{
"ui_config": {
"enabled": true
},
"client_addr": "0.0.0.0",
"bind_addr": "10.253.253.100",
"retry_join": ["10.253.253.100"],
"server": true,
"limits": {
"http_max_conns_per_client": 8192,
"rpc_max_conns_per_client": 2048
},
"bootstrap_expect": 1
}
```

### Operating system and Environment details

Linux 5.4.0-198-generic #218-Ubuntu SMP Fri Sep 27 20:18:53 UTC 2024 x86_64 x86_64 x86_64 GNU/Linux

Start in agent/checks/check.go at the CheckUDP Start() and check() functions, especially the referenced read-timeout handling. Reproduce the issue with a 1s UDP health check by stopping the responder after one successful check. Done means the check continues polling and changes from passing when the UDP endpoint becomes unavailable.

Written by the indexing model from the issue text.