semantic-layer edit: use the metastore PATCH instead of a whole-record PUT #833

Follow-up from the review of #778 (soustruh, 2026-10-06).

semantic-layer edit (edit_simple, edit_metric_with_cascade in services/_semantic_layer_crud.py) copies every current attribute, applies the change and sends a whole-record put_item. A change another caller makes to a different key between the read and the PUT is overwritten.

The metastore has a PATCH /api/v1/repository/{type}/{id} that merges only the top-level data keys of the request (simpleJSONMerge), in one transaction under a row lock. #778 adds MetastoreClient.patch_item(item_type, item_id, *, name=None, data=None).

Proposed:

• semantic-layer edit: send patch_item with only the changed keys (for the metric-rename cascade: only metrics). The reads stay: they resolve the item by name and drive the cascade.
• import --overwrite and promote keep put_item: they must make the target equal to the source, and a PATCH would keep keys that exist only in the target.
• Note: the PATCH validates only the keys it receives, never the merged record, so the merged item must still be validated client-side.
• Correct the 0.41.10 changelog statement that "the metastore has no PATCH endpoint" where it is relied on in docs/comments (the route exists in go-monorepo since at least b84391fa, 2025-07-02).

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