Bug fix on clear_number migration script to properly set the default value

[sungwy]

closes: #34332
related: #34126

As @abhishekbhakat reported, the merged method of enabling 'default=0' produces an autogenerated sql that causes issues on the db, with nullable=False. 'server_default=0' should be used instead here, but server_default=0 results in the creation of a default DB constraint on MSSQL, which cannot be named deterministically, and hence cannot be removed in the downgrade.

sqlalchemy.exc.ProgrammingError: (pyodbc.ProgrammingError) ('42000', "[42000] [Microsoft][ODBC Driver 18 for SQL Server][SQL Server]Incorrect syntax near 'constraint'. (102) (SQLExecDirectW)")
[SQL: declare @const_name varchar(256)
select @const_name = QUOTENAME([name]) from sys.default_constraints
where parent_object_id = object_id('dag_run')
and col_name(parent_object_id, parent_column_id) = 'clear_number'
exec('alter table dag_run drop constraint ' + @const_name)]

[jscheffl]

Can you add a description what this PR is for? Any relation to a bug or feature?
Otherwise if you experiment-around, maybe mark the PR as DRAFT?

[sungwy]

Can you add a description what this PR is for? Any relation to a bug or feature? Otherwise if you experiment-around, maybe mark the PR as DRAFT?

Sorry about that @jens-scheffler-bosch ! made the requested change.

[sungwy]

tagging @ephraimbuddy @abhishekbhakat and @uranusjr for review

[phanikumv]

Can you fix the conflicts please?

[sungwy]

@phanikumv I've resolved the merge conflict, and tests were passing but 'Mergeable' check was stuck 'In Progress' - triggered another CI run by updating the branch

[boctorj]

Thanks for fixing the issue but this doesn't fixed mssql. Is there a reason why you forked the code to support this?
alter_column can take a 'server_default' value as well.

[abhishekbhakat]

Discussed here: https://github.com/apache/airflow/pull/34335/files/ae9920a4be57acf30ee6a7923d3a1e4708a1c36d

Gist: Issue was with downgrade.