[Bug]: Bad connection string health check shows "threshold exceeded" instead of connection error

When the data source connection string points to a non-existent server, the health check reports `"The threshold for executing the request has exceeded."` instead of a connection error.

Operators monitoring health endpoints cannot determine the root cause of an Unhealthy status. A threshold exceeded vs. connection failure require very different remediation actions.

## Expected

The health check should report a connection-related exception, e.g.:
```json
{
"status": "Unhealthy",
"name": "primary-sql-db",
"exception": "A network-related or instance-specific error occurred while establishing a connection to SQL Server.",
"tags": ["data-source"]
}
```

## Actual

```json
{
"status": "Unhealthy",
"name": "primary-sql-db",
"exception": "The threshold for executing the request has exceeded.",
"tags": ["data-source"],
"data": {"response-ms": 137, "threshold-ms": 1500}
}
```

The actual connection failure gets swallowed and replaced with a generic threshold message. This makes it impossible to distinguish between "database is slow" and "database is unreachable" from the health response alone.

Beginne damit, den Einstiegspunkt des Health-Checks für die Datenquelle und die Verarbeitung zu lokalisieren, die eine fehlgeschlagene SQL-Verbindung in die Schwellenwertmeldung umwandelt. Reproduziere anschließend den Fall eines nicht existierenden Servers und verifiziere dann, dass die Health-Antwort eine verbindungsbezogene Exception beibehält, während Schwellenwertdaten für tatsächlich langsame Requests erhalten bleiben.

Vom Indexierungsmodell aus dem Issue-Text verfasst.