[Subtask] feat(mcp-server): read tools for table partitions (list/get)

[bharos]

Describe the subtask

Follow-up to #11574 / #11693. The MCP server already exposes
list_statistics_for_partition, but there is no tool to discover partition
names/infos, so an agent has no way to obtain the from/to partition inputs
that tool requires. Add read-only partition tools following the established
pattern (interface → plain REST client impl → FastMCP tool → mock → unit tests),
with authorization enforced by Gravitino.

Proposed tools:

• list_of_partitions(catalog, schema, table, details=false) — GET .../tables/{table}/partitions (names, or full infos when details=true).
• get_partition(catalog, schema, table, partition) — GET .../tables/{table}/partitions/{partition}.

Scope: read-only (list/get). Partition add/drop (write) can be a later subtask.

The backend REST already exists (PartitionOperations, both GETs gated by
LOAD_TABLE_AUTHORIZATION_EXPRESSION), so authorization-scoped discovery is
inherited automatically — no filtering logic added in the MCP layer.

Parent issue

#11573

[bharos]

cc @yuqi1129 @jerryshao , does this fit the EPIC scope? I'll create a PR once you confirm.