feat(semantic-layer): kbagent semantic-layer schema — JSON schema per object type (port get_semantic_schema)

[padak]

Part of #390.

Summary

Add kbagent semantic-layer schema --type … to fetch the JSON schema of semantic object types, porting the get_semantic_schema MCP tool.

MCP source (keboola-mcp-server @ agent-v1.60.0)

• Tool: src/keboola_mcp_server/tools/semantic/tools.py (get_semantic_schema) — for each requested type calls metastore_client.get_schema(type).
• Client: clients/metastore.py get_schema(object_type, version=None) → GET api/v1/schema/{object_type} (optional /{version}). Returns the raw JSON Schema the server emits, wrapped as {semantic_type, schema}.
• Important: the schema is server-fetched, not static Pydantic — so it always matches the deployed metastore version.

What kbagent already has

• metastore_client.py MetastoreClient uses the same base URL (metastore.{suffix}) and auth (X-StorageApi-Token), but exposes only repository CRUD verbs (list_items/get_item/post_item/put_item/delete_item). No get_schema, no /api/v1/schema/ call.
• The --type notion already exists (semantic-layer show --type, search-context --type) via TYPE_ALIAS in semantic_layer_service.py — reuse for the singular→wire mapping (add model).

Proposed command

kbagent semantic-layer schema --project P --type metric[,dataset,relationship,constraint,glossary,model] [--all]

Implementation sketch (3-layer)

• L3 MetastoreClient.get_schema(item_type) -> dict → GET /api/v1/schema/{item_type}.
• L2 SemanticLayerService.get_schema(alias, types) → reuse TYPE_ALIAS, fan out per type (the service already uses ThreadPoolExecutor, cf. _fetch_children_parallel), return {project, schemas: [{type, schema}]}.
• L1 thin command; --json raw schemas; human = per-type panel/tree.

Effort: S

Acceptance criteria

• kbagent semantic-layer schema --type metric,dataset returns each type's JSON schema.
• --json emits {project, schemas:[{type, schema}]}.
• Unit test + E2E test.
• Doc-sync surfaces per Epic: Port keboola-mcp-server agent tools into native kbagent commands #390 checklist.

Risks / open questions

• Fetch live — do NOT hand-roll static schemas (they would drift the moment the metastore evolves). Mirror upstream.
• --project is required (schema fetched from that project's stack) even though schemas are stack-global — acceptable, matches all other semantic-layer subcommands.
• Optional: support ?version= (upstream has it; low cost).