feat(mito): backfill partition expr on region open

[zhongzc]

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What's changed and what's your intention?

Older regions didn’t persist partition_expr in engine metadata; new regions do. To unify behavior and support online upgrade, we backfill partition_expr on region open and persist it to the manifest.

Added on-open backfill to persist partition_expr for legacy regions.

Backfill runs only when metadata.partition_expr is None; no impact on regions already carrying the field.

Manual Test

SQL on release/0.15

CREATE TABLE my_table (
  a INT PRIMARY KEY,
  b STRING,
  ts TIMESTAMP TIME INDEX,
)
PARTITION ON COLUMNS (a) (
  a < 1000,
  a >= 1000 AND a < 2000,
  a >= 2000
);

manifests:

{"actions":[{"Change":{"metadata":{"column_metadatas":[{"column_schema":{"name":"a","data_type":{"Int32":{}},"is_nullable":true,"is_time_index":false,"default_constraint":null,"metadata":{}},"semantic_type":"Tag","column_id":0},{"column_schema":{"name":"b","data_type":{"String":null},"is_nullable":true,"is_time_index":false,"default_constraint":null,"metadata":{}},"semantic_type":"Field","column_id":1},{"column_schema":{"name":"ts","data_type":{"Timestamp":{"Millisecond":null}},"is_nullable":false,"is_time_index":true,"default_constraint":null,"metadata":{"greptime:time_index":"true"}},"semantic_type":"Timestamp","column_id":2}],"primary_key":[0],"region_id":4398046511104,"schema_version":0,"primary_key_encoding":"dense"}}}]}

Restart with this patch, manifests:

{"actions":[{"Change":{"metadata":{"column_metadatas":[{"column_schema":{"name":"a","data_type":{"Int32":{}},"is_nullable":true,"is_time_index":false,"default_constraint":null,"metadata":{}},"semantic_type":"Tag","column_id":0},{"column_schema":{"name":"b","data_type":{"String":null},"is_nullable":true,"is_time_index":false,"default_constraint":null,"metadata":{}},"semantic_type":"Field","column_id":1},{"column_schema":{"name":"ts","data_type":{"Timestamp":{"Millisecond":null}},"is_nullable":false,"is_time_index":true,"default_constraint":null,"metadata":{"greptime:time_index":"true"}},"semantic_type":"Timestamp","column_id":2}],"primary_key":[0],"region_id":4398046511104,"schema_version":0,"primary_key_encoding":"dense","partition_expr":"{\"Expr\":{\"lhs\":{\"Column\":\"a\"},\"op\":\"Lt\",\"rhs\":{\"Value\":{\"Int32\":1000}}}}"}}}]}

PR Checklist

Please convert it to a draft if some of the following conditions are not met.

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.
• API changes are backward compatible.
• Schema or data changes are backward compatible.

[fengjiachun]

LGTM

[WenyXu]

Rest LGTM