Bug: ELASTICSEARCH_INDEX with multiple indices breaks <index>_locations resolution

## Problem
This MCP assumes a split-index model: `` for chunks + `_locations` for file/line metadata.

If `ELASTICSEARCH_INDEX` is set to a comma-separated list (e.g. `kibana,elasticsearch`), we pass that string directly as the Elasticsearch `index` parameter (multi-index search), but we also derive the locations index as `${baseIndex}_locations`.

That produces an invalid locations index name like `kibana,elasticsearch_locations` instead of querying `kibana_locations` and `elasticsearch_locations`.

## Impact
- `semantic_code_search` location join breaks (errors or returns empty `locations`)
- `read_file`, `map_symbols_by_query`, `symbol_analysis` break because they query the derived locations index

## Repro
1. Set `ELASTICSEARCH_INDEX=kibana,elasticsearch`
2. Call any tool that needs locations (e.g. `read_file`, or `semantic_code_search` which returns `locations`)
3. Observe an `index_not_found_exception` for `kibana,elasticsearch_locations` (or missing/empty location data)

## Code pointers
- `src/utils/elasticsearch.ts`: `getLocationsIndexName()`
- Tools that call it: `src/mcp_server/tools/semantic_code_search.ts`, `read_file.ts`, `map_symbols_by_query.ts`, `symbol_analysis.ts`

## Expected
Either:
- Support multi-index properly by resolving locations per-hit-index (requires carrying index provenance through the pipeline), or
- Reject comma-separated `ELASTICSEARCH_INDEX` early with a clear error explaining the split-index model limitation and how to configure indexes.