Add Spec → LinkML → Spec round-trip tests (CSRMatrix / base.yaml)

[rly]

Part of the "Read and write LinkML schema for HDMF (CSRMatrix / base.yaml)" epic. Depends on the writer issue and the LinkML reader issue.

Verify that the writer and reader are mutually consistent: writing a Spec to LinkML and reading it back with the reader reproduces the original Spec, for CSRMatrix and the base types. This closes the loop on the conventions, confirming the read and write directions agree on the same representation.

Acceptance criteria

• Round-trip test: for CSRMatrix and the base types, Spec → LinkML → Spec reproduces the original Spec.
• Tests cover the round-trip at HDMF's usual coverage standard.