[Gap]: Still no MCP health checks despite MCP being enabled

The health response includes `"mcp": true` in the `configuration` section, but there are no MCP-specific health checks. Only REST and GraphQL entity checks appear.

## Expected

If MCP is enabled and entities are exposed via MCP, MCP endpoint health checks should appear in the `checks` array with tags like `["mcp", "list_tools"]`, similar to how REST and GraphQL each get their own checks.

| Check | Tags | What it validates | Notes |
|-------|------|-------------------|-------|
| **list_tools** | `["mcp", "list_tools"]` | Server up, tools registered, expected count based on what is enabled in the config | Baseline. If this fails, skip the rest. |
| **describe_entities** | `["mcp", "describe_entities"]` | Schema/metadata generation, pagination based on what is enabled in the config | Catches schema generation bugs. Entity count in `data` field is a good signal. |
| **read_entity** (per entity) | `["mcp", "read_entity"]` | Full data retrieval path, top 1 | Mirrors REST/GraphQL per-entity checks. Validates JSON-RPC param binding + result serialization. |
| **aggregate_entity** (per entity) | `["mcp", "aggregate_entity"]` | Aggregation code path, COUNT(*) | Genuinely different query generation than read. Cheap query, high signal. |

** MCP checks would ideally reuse one SSE connection across all MCP checks within a single health evaluation. That keeps the overhead comparable to the REST/GraphQL checks combined rather than multiplied by connection setup cost.

## Actual

The `checks` array only contains:
- Data source checks (`tags: ["data-source"]`)
- REST entity checks (`tags: ["rest", "endpoint"]`)
- GraphQL entity checks (`tags: ["graphql", "endpoint"]`)

No MCP checks exist. Combined with the missing MCP metrics from the OTEL report, MCP remains the least observable API surface.

Finde die Health-Response und die bestehende Generierung der REST- und GraphQL-Entity-Checks und verfolge anschließend, wie aktivierte Entities aus der Konfiguration gelesen werden. Füge MCP-Checks für list_tools, describe_entities, read_entity und aggregate_entity hinzu, einschließlich der angegebenen Tags und des Baseline-Verhaltens, und berücksichtige dabei die Wiederverwendung einer SSE-Verbindung. Als erledigt gilt die Aufgabe, wenn die Checks nur erscheinen, wenn MCP-Entities aktiviert sind, und die aufgeführten Pfade validieren.

Vom Indexierungsmodell aus dem Issue-Text verfasst.