Add a structured provider/model health probe

[gkennos]

Pre-flight

• I searched existing issues and this has not been requested before.

Problem or motivation

Groundworkers currently needs to verify that a configured model provider is reachable and usable before starting embedding population.

ModelBackend.is_available() currently returns only bool and suppresses the underlying exception.

Without this, callers receive a generic “provider unavailable” result even when the endpoint is healthy but the configured URL is incompatible with the provider adapter.

We need to know downstream:

• endpoint unreachable;
• provider reachable but model missing;
• model not configured for embeddings;
• embedding dimension lookup failure;
• provider responding through an incompatible endpoint path.

Proposed solution

Add a provider-neutral structured probe to omop-llm, for example:

result = backend.probe()

The result should expose enough information for callers to provide actionable setup diagnostics:

e.g.

@dataclass(frozen=True)
class ProbeResult:
    reachable: bool
    model_available: bool | None
    embeddings_supported: bool
    dimensions: int | None
    failure: ProbeFailure | None