[Subtask] Complete V3 type compatibility for the OceanBase catalog

### Describe the subtask

Complete and document the five-family compatibility contract for OceanBase CE 4.2.1 in MySQL mode.

**Compatibility outcomes**

- **Nanosecond timestamps:** Reject precision above 6.
- **Variant:** Reject because OceanBase JSON is a narrower contract.
- **Unknown/Null:** Reject.
- **Geometry:** Reject because current reload loses CRS and SRID.
- **Geography:** Reject because OceanBase has no faithful geography semantics.

**Acceptance**

- Reproduce all five families through the connector's public create/conversion path and its alter/load paths where supported.
- A supported mapping must round-trip without losing type identity or parameters.
- An unsupported mapping must return `IllegalArgumentException` before external metadata mutation; no fallback to a narrower or opaque type is allowed.
- Cover nested types where the connector supports complex columns.
- Add focused converter/operation tests and an integration or no-side-effect test appropriate to the connector.
- Update the connector documentation with a five-family compatibility matrix and rejection rationale.
- Keep implementation commits atomic by connector and type family.

**Connector-specific verification**

Reproduce all five creates against Docker OceanBase and verify each rejects and leaves no table. Cover converter, SQL-generation, module, and formatting checks.

**Documentation**

`docs/jdbc-oceanbase-catalog.md`

### Parent issue

#12056