InfluxDB3 Core: DiskManager disabled prevents sort spilling, causes ResourcesExhausted on multi-day queries

## Problem

InfluxDB3 Core hardcodes `DiskManagerMode::Disabled` in the DataFusion `RuntimeEnv` configuration. This prevents the `ExternalSorter` from spilling to disk when the memory pool is exhausted.

The `DeduplicateExec` operator requires `Distribution::SinglePartition`, forcing a `SortPreservingMergeExec` to merge ALL file partitions into a single stream. With `max_parquet_fanout` greater than the file count, each file gets its own partition. For multi-day queries spanning 1000+ files, the merge operator holds 1000+ cursors simultaneously — all marked `can spill: false`.

This causes errors like:

```
ResourcesExhausted: Additional allocation failed with top memory consumers (bytes):
SortPreservingMergeExec(can spill: false): ...
```

...even with 100+ GB memory pools.

The `ExternalSorter` operators DO report `can spill: true`, but cannot actually spill because the `DiskManager` is disabled at the runtime level.

## Request

Make `DiskManager` configurable via CLI flag or environment variable (e.g., `--disk-manager-mode=auto` or `INFLUXDB3_DISK_MANAGER_MODE`). The default could remain disabled for backwards compatibility.

## Workaround

Reducing `max_parquet_fanout` below the file count forces DataFusion to use `target_partitions` (e.g., 80) instead of per-file partitions, reducing merge cursor count. However, this disables the pre-sorted file optimization and adds sort overhead.

## Environment

- InfluxDB3 Core (not Enterprise)
- DataFusion query execution

**Note:** Analysis performed using Claude Code based on source code review and production error patterns.