Adds the missing server_id system variable to MatrixOne so ODBC clients can read @@server_id during connection initialization without error. Also adds regression coverage in the distributed system variable tests. This prevents Tableau/ODBC connections from failing with “system variable does not exist.”

Approved by: @fengttt, @heni02, @XuPeng-SH

…stmt null json panic (matrixorigin#23864)

When a prepared statement inserts NULL into a JSON column multiple times, the second execution causes a panic during SELECT:

```
panic runtime error: index out of range [0] with length 0
ByteJson.Unmarshal (bytejson.go:96)
DecodeJson (encoding.go:93)
ColumnSlices.GetStringBased (output.go:538)
```

**Reproduce:**
```sql
CREATE TABLE test_ps (id INT, meta JSON);
PREPARE stmt FROM 'INSERT INTO test_ps (id, meta) VALUES (?, ?)';
SET @A=1, @b=NULL; EXECUTE stmt USING @A, @b;
SET @A=2, @b=NULL; EXECUTE stmt USING @A, @b;
DEALLOCATE PREPARE stmt;
SELECT meta FROM test_ps where id = 1;  -- ok
SELECT meta FROM test_ps where id = 2;  -- panic
```

**Root cause:** `FunctionExpressionExecutor.doFold()` calls `ToConst()` on the result vector after constant folding. On the second execution, `PreExtendAndReset()` calls `ResetWithSameType()` which clears nsp (null bitmap) but preserves the CONSTANT class. When `FunctionResult.AppendBytes(nil, true)` is called on a CONSTANT vector, it skips setting the null marker entirely, resulting in a non-null empty byte slice being stored instead of NULL.

**Fix:** Reset vector class to FLAT in `ResetWithSameType()`, so the vector is in a clean state for re-population. `doFold()` will call `ToConst()` again after evaluation if needed.

Approved by: @heni02, @aunjgr

This PR fixes a missing `MYSQL_TYPE_JSON` branch in the prepared-statement binary execute parser on `main`.

Without this change, `COM_STMT_EXECUTE` falls through to `unsupport parameter type` when a client sends a JSON-typed parameter through the binary protocol.

Focused validation:
- `go test ./pkg/frontend -run 'TestParseExecuteDataWithJSONParam'`
- `go test ./pkg/frontend -run 'TestParseExecuteDataWithJSONParam|Test_StripPassword'`

Approved by: @LeftHandCold

This PR reclaims prepared-statement binary parameter state after `COM_STMT_EXECUTE` and on malformed execute cleanup.

It fixes two closely related problems on `main`:
1. repeated binary executes with varlen parameters reused the same params vector and kept appending to its area, growing session memory until statement close;
2. malformed `COM_STMT_EXECUTE` packets could leave a partially populated params vector attached to the prepared statement because the parse-error path dropped the statement pointer before cleanup.

Focused validation:
- `go test ./pkg/frontend -run 'TestPrepareStmtClearBinaryParamStateReleasesParamArea|Test_ExecRequestStmtExecuteErrorClearsPreparedParamState'`
- `go test ./pkg/frontend -run 'TestPrepareStmtClearBinaryParamStateReleasesParamArea|Test_ExecRequestStmtExecuteErrorClearsPreparedParamState|Test_parseStmtSendLongData'`

Approved by: @LeftHandCold

This PR fixes three categories of issues:

- Repeated COM_STMT_SEND_LONG_DATA chunks for the same parameter now **append** instead of overwriting, matching MySQL protocol semantics.
- Truncated COM_STMT_EXECUTE packets now return a malformed-packet error instead of risking a panic while reading the new parameter bound flag.

- PreInsert.refreshAutoIncrementTableID() wrote directly to the shared *plan.TableDef.TblId during Prepare().
- MultiUpdate.Prepare() read the same TblId concurrently in a different ants worker pool goroutine.
- **Fix**: Store the refreshed TblId in a local container field (ctr.tblId) instead of mutating the shared plan object.

- PartitionInsert and PartitionDelete temporarily mutated the shared ObjRef.ObjName during partition iteration (save/restore via defer) -- replaced with stack-local copies.
- PartitionMultiUpdate.writeTable() directly mutated shared MultiUpdateCtx entries -- now uses clone() to match the writeS3 code path.
- Also fixed a pre-existing copy-paste bug in MultiUpdateCtx.clone() where PartitionCols was incorrectly copying DeleteCols.

Approved by: @ouyuanning

This PR hardens prepared-statement binary protocol cleanup on `main`.

It fixes three closely related problems in the frontend prepare path:
1. `PrepareStmt.Close()` now keeps a valid process context for freeing binary parameter vectors after binary parsing.
2. `COM_STMT_RESET` / `reset prepare` now clears accumulated binary prepared-statement state instead of acting like a no-op.
3. `COM_STMT_CLOSE`, `COM_STMT_RESET`, and malformed `COM_STMT_EXECUTE` error paths now cleanly return and clean up instead of dereferencing nil state or leaving dirty binary state behind.

Focused validation:
- `go test ./pkg/frontend -run 'Test_doResetClearsPreparedBinaryState|Test_ExecRequestPrepareCommandMissingStmt|Test_ExecRequestStmtExecuteErrorClearsPreparedBinaryState|TestPrepareStmtCloseAfterBinaryParamParsing|TestMysqlProtocolImpl_Close|Test_parseStmtSendLongData'`

Approved by: @LeftHandCold