[Bug]: 3.0-dev lacks SQL syntax and operator compatibility updates

[LeftHandCold]

Is there an existing issue for the same bug?

• I have checked the existing issues.

Branch Name

3.0-dev

Commit ID

d1e78d8

Other Environment Information

- Hardware parameters: N/A (branch-compare audit)
- OS type: Darwin
- Others:
  - Compared `base/main` vs `base/3.0-dev`
  - Audit window: 2025-10-01 .. 2026-04-24

Actual Behavior

3.0-dev still lacks several user-visible SQL syntax and operator compatibility updates that are already on main. After removing overlaps with other issues, the remaining items in this bucket are: MySQL JSON arrow operators (->, ->>), the null-safe equal operator (<=>), and multi-table DROP TABLE.

Expected Behavior

3.0-dev should accept the same SQL syntax/operator surface as main for these features, instead of diverging at parser/binder level.

Steps to Reproduce

1. Compare `base/main` and `base/3.0-dev` for PRs `#23717`, `#23599`, `#23716`.
2. Verify the missing implementation/test surfaces on `3.0-dev`:
   - `git grep -nE '->>|->' base/3.0-dev -- pkg/sql/parsers/dialect/mysql/mysql_sql.y` -> no match
   - `git grep -n '<=>' base/3.0-dev -- pkg/sql/parsers/dialect/mysql/mysql_sql.y` -> no match
   - `git cat-file -e base/3.0-dev:test/distributed/cases/ddl/drop_table_multiple.sql` -> missing
   - `git cat-file -e base/3.0-dev:test/distributed/cases/function/func_json_arrow.sql` -> missing
   - `git cat-file -e base/3.0-dev:test/distributed/cases/function/func_null_safe_equal.sql` -> missing
3. Try representative SQL on `3.0-dev`, for example:
   - `select doc->'$.a', doc->>'$.a' from t;`
   - `select 1 <=> null;`
   - `drop table t1, t2;`

Additional information

Main-only reference commits:

• 0215e33ff1 feat: MySQL JSON operators -> and ->> plus JSON to numeric cast (#23717)
• 612da2cc5a feat: implement null-safe equal operator (<=>) (#23599)
• f3cab27f13 implement drop multi tables drop (#23716)

Generated columns are no longer included in this issue because they already have a dedicated existing issue: #23120.