[Bug]: add-telemetry overwrites Previous Telemetry Settings

## Expected
Second call should only modify OTEL settings, preserving App Insights settings.

### North star

```
dab configure --runtime.telemetry.application-insights.enabled
dab configure --runtime.telemetry.application-insights.connection-string
dab configure --runtime.telemetry.open-telemetry.enabled
dab configure --runtime.telemetry.open-telemetry.endpoint
dab configure --runtime.telemetry.open-telemetry.headers
dab configure --runtime.telemetry.open-telemetry.exporter-protocol
dab configure --runtime.telemetry.open-telemetry.service-name
```

So that we can **deprecate** `add-telemetry`

## Actual
Second call resets app-insights enabled to false and removes connection-string.

## Workaround
Configure all telemetry providers in a single add-telemetry call.

## Details

Running dab add-telemetry --app-insights-enabled true then dab add-telemetry --otel-enabled true results in app-insights being reset to false and its connection-string being removed. Each call replaces the entire telemetry block rather than merging.

```bash
dab init --database-type mssql --connection-string "test" --config bug6.json

dab add-telemetry --app-insights-enabled true --app-insights-conn-string "InstrumentationKey=test123" --config bug6.json
# Config: application-insights.enabled = true, connection-string = "InstrumentationKey=test123" ✓

dab add-telemetry --otel-enabled true --otel-endpoint "http://localhost:4317" --config bug6.json
# Config: application-insights.enabled = false ✗ (overwritten)
# Config: application-insights.connection-string removed ✗
# Config: open-telemetry correctly set ✓