Overview

Give the DB Integration & Observability (DINO) team ownership of the jstests/core/diagnostics test directory, replacing the previous mixed/unowned state.

Background

Tests under jstests/core/diagnostics cover diagnostics and observability functionality owned by DINO, but the directory was not assigned to the team in CODEOWNERS, so changes there did not reliably route to the owning team for review.

Scope of Work

* Add an OWNERS.yml for jstests/core/diagnostics mapping to 10gen/query-integration-observability.
* Regenerate the corresponding entry in .github/CODEOWNERS.

Acceptance Criteria

* .github/CODEOWNERS routes /jstests/core/diagnostics/** to @10gen/query-integration-observability.
* Existing, more specific ownership entries (e.g. jstests/core/diagnostics/datasize) are preserved.