refactor: rm operator+ from StringRef

[DSFans2014]

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

operator+ in StringRef may has memory leak

• What is the current behavior? (You can also link to an open issue here)

• What is the new behavior (if this is a feature change)?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.35%. Comparing base (06c2b67) to head (e053596).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #4131      +/-   ##
============================================
+ Coverage     75.33%   75.35%   +0.02%     
  Complexity      711      711              
============================================
  Files           763      763              
  Lines        143753   143742      -11     
  Branches       1480     1480              
============================================
+ Hits         108291   108314      +23     
+ Misses        35159    35125      -34     
  Partials        303      303              

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[github-actions]

SDK Test Report

102 files  ±0  102 suites  ±0   2m 12s ⏱️ +6s
359 tests ±0  345 ✅ ±0  14 💤 ±0  0 ❌ ±0 
487 runs  ±0  473 ✅ ±0  14 💤 ±0  0 ❌ ±0 

Results for commit e053596. ± Comparison against base commit 087381d.

This pull request removes 30 and adds 9 tests. Note that renamed tests count towards both.

  PARTITION BY db1.t1.col2 ORDER BY db1.t1.col1
  PARTITION BY t1.col2 ORDER BY t1.col1
  ROWS_RANGE BETWEEN 3 PRECEDING AND CURRENT ROW
 ) limit 10;](1)
 ) limit 10;](2)
 ) limit 10;](3)
 FROM db1.t1
 FROM t1
 WINDOW w1 AS (
 last join db2.t2 order by db2.t2.col1
…

com._4paradigm.hybridse.sdk.SqlEngineTest ‑ sqlLastJoinWithMultipleDB[,  SELECT sum(db1.t1.col1) over w1 as sum_t1_col1, db2.t2.str1 as t2_str1
 FROM db1.t1
 last join db2.t2 order by db2.t2.col1
 on db1.t1.col1 = db2.t2.col1 and db1.t1.col2 = db2.t2.col0
 WINDOW w1 AS (
  PARTITION BY db1.t1.col2 ORDER BY db1.t1.col1
  ROWS_RANGE BETWEEN 3 PRECEDING AND CURRENT ROW
 ) limit 10;](2)
com._4paradigm.hybridse.sdk.SqlEngineTest ‑ sqlLastJoinWithMultipleDB[db1,  SELECT sum(t1.col1) over w1 as sum_t1_col1, db2.t2.str1 as t2_str1
 FROM t1
 last join db2.t2 order by db2.t2.col1
 on t1.col1 = db2.t2.col1 and t1.col2 = db2.t2.col0
 WINDOW w1 AS (
  PARTITION BY t1.col2 ORDER BY t1.col1
  ROWS_RANGE BETWEEN 3 PRECEDING AND CURRENT ROW
 ) limit 10;](1)
com._4paradigm.hybridse.sdk.SqlEngineTest ‑ sqlLastJoinWithMultipleDB[null,  SELECT sum(db1.t1.col1) over w1 as sum_t1_col1, db2.t2.str1 as t2_str1
 FROM db1.t1
 last join db2.t2 order by db2.t2.col1
 on db1.t1.col1 = db2.t2.col1 and db1.t1.col2 = db2.t2.col0
 WINDOW w1 AS (
  PARTITION BY db1.t1.col2 ORDER BY db1.t1.col1
  ROWS_RANGE BETWEEN 3 PRECEDING AND CURRENT ROW
 ) limit 10;](3)
com._4paradigm.hybridse.sdk.SqlEngineTest ‑ sqlMultipleDBErrorTest[, SELECT db2.t2.str1 as t2_str1
 FROM t1
 last join db2.t2 order by db2.t2.col1
 on t1.col1 = db2.t2.col1 and t1.col2 = db2.t2.col0;
, SQL parse error: Fail to transform data provider op: table t1 not exists in database []](4)
com._4paradigm.hybridse.sdk.SqlEngineTest ‑ sqlMultipleDBErrorTest[db1, SELECT db1.t2.str1 as t2_str1
 FROM t1
 last join db2.t2 order by db2.t2.col1
 on t1.col1 = db2.t2.col1 and t1.col2 = db2.t2.col0;
, SQL parse error: Column Not found: db1.t2.str1](2)
com._4paradigm.hybridse.sdk.SqlEngineTest ‑ sqlMultipleDBErrorTest[db1, SELECT db2.t2.str1 as t2_str1
 FROM t1
 last join db2.t2 order by db2.t2.col1
 on t1.col1 = t2.col1 and t1.col2 = db2.t2.col0;
, SQL parse error: Column Not found: .t2.col1](3)
com._4paradigm.hybridse.sdk.SqlEngineTest ‑ sqlMultipleDBErrorTest[db1, SELECT t2.str1 as t2_str1
 FROM t1
 last join db2.t2 order by db2.t2.col1
 on t1.col1 = db2.t2.col1 and t1.col2 = db2.t2.col0;
, SQL parse error: Column Not found: .t2.str1](1)
com._4paradigm.hybridse.sdk.SqlEngineTest ‑ sqlMultipleDBErrorTest[null, SELECT db2.t2.str1 as t2_str1
 FROM t1
 last join db2.t2 order by db2.t2.col1
 on t1.col1 = db2.t2.col1 and t1.col2 = db2.t2.col0;
, SQL parse error: Fail to transform data provider op: table t1 not exists in database []](5)
com._4paradigm.hybridse.sdk.SqlEngineTest ‑ sqlWindowLastJoin[ SELECT sum(t1.col1) over w1 as sum_t1_col1, t2.str1 as t2_str1
 FROM t1
 last join t2 order by t2.col1
 on t1.col1 = t2.col1 and t1.col2 = t2.col0
 WINDOW w1 AS (
  PARTITION BY t1.col2 ORDER BY t1.col1
  ROWS_RANGE BETWEEN 3 PRECEDING AND CURRENT ROW
 ) limit 10;](1)