fix: return 503 when scheduler has no available nodes

When the scheduler cannot find any healthy node to serve a model —
all nodes are full and eviction cannot free a slot, or a node_selector
excludes every candidate — the error fell through to 500. A 500 tells
clients something is broken when the condition is transient and
retryable.

The router now wraps these errors with a new ErrNoAvailableNodes
sentinel. The HTTP error handler maps it to 503 via applyNoAvailableNodes,
following the same pattern as applyBackendAdmission (429). Unrelated
scheduler errors (DB timeouts, registry lookups) still return 500.

Three return sites are wrapped:
- resolveSelectorCandidates: selector matches zero healthy nodes
- scheduleNewModel eviction-busy: all models have in-flight requests
- scheduleNewModel eviction-failed: eviction itself errored

The existing scheduleAndLoad wrapper ("no available nodes: %w") preserves
the sentinel through the chain via errors.Is, as does ModelRouterAdapter.

Assisted-by: AGENT:regolo/glm5.2 [TOOL]

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

docs/content/reference/runtime-errors.md

@@ -21,6 +21,7 @@ The left column is the literal string as it appears in the LocalAI server log (o
 | `grpc service not ready` | The backend process was spawned but its gRPC server did not become healthy in time (slow start, crash on startup, or the process died while loading). When a local backend has already exited, the error includes its exit code and last stderr line. | Use the included stderr diagnostic when present; otherwise check the log lines just above. A crash here often means out of memory, a missing shared library, or an incompatible CPU (see `SIGILL`). Increase available RAM/VRAM or pick a smaller quantization. |
 | `failed to load model: ...` | Returned by the load endpoints and several feature paths (voice, realtime, audio transform) when the model config could not be resolved or the backend load failed. | Confirm the model name exists (`local-ai models list`) and its YAML is valid. The trailing text carries the specific reason. |
 | HTTP `503` with a `Retry-After` header, after a load failed | Model-load failure cooldown. After a model fails to load, LocalAI refuses new load attempts for that model for a short window so a client that keeps polling a broken model does not respawn a crashing backend on every request. The window starts at `--model-load-failure-cooldown` (default `10s`) and doubles per consecutive failure up to 5m; it resets on the first success. | Fix the underlying load failure (see the rows above), then wait out the `Retry-After` seconds before retrying, or restart LocalAI to clear the cooldown. Set `--model-load-failure-cooldown 0` (or `LOCALAI_MODEL_LOAD_FAILURE_COOLDOWN=0`) to disable the cooldown entirely. See {{% relref "reference/cli-reference" %}}. |
+| HTTP `503` with `no available nodes` or `no healthy nodes match selector` | The scheduler could not find any healthy node to serve the model. All nodes are full and eviction cannot free a slot, or a `node_selector` in the model's scheduling config excludes every candidate. | Retry after a node becomes available or an in-flight request completes and frees a slot. In a cluster, add nodes or replicas. If a selector is set, confirm at least one healthy node matches it. |
 | HTTP `429` with a `Retry-After` header, under load | Per-model concurrency limit reached. When a model config sets a `MaxConcurrent` limit, extra requests are rejected with `429` and a `Retry-After` (whole seconds, floor 1) instead of queueing. | Retry after the advised delay, raise the model's concurrency limit, or run more replicas. |
 | HTTP `429` when backend inference is saturated | The process-wide `--max-concurrent-backend-requests` backend-execution ceiling is full. This protects inference and in-flight backend-trace memory without blocking UI or administrative endpoints. | Retry after the advised delay, reduce inference concurrency, raise the limit if the host has capacity, or add replicas. |
 | `invalid pitch` (with CUDA) | The prompt exceeded the model's context size. | Reduce the prompt length, or raise the model's context size (`context_size:` in the model YAML). |