remove mock.Context usage in production code

[lcwangchao]

Enhancement

It's better to remove the mock.Context usages in TiDB production code. The reason is that mock.Context is not designed for production code. It is designed for testing, that means some values in it are not reasonable in production and some values will be modified by the developer without any care.

We can replace some code that uses mock.Context with some "static" context we introduced before such as StaticExprContext or DistSQLContext

Tasks

• ddl: remove mock.Context ref in ddl/session/session_pool.go #53378
• ddl: introduce newReorgExprCtx to replace mock.Context usage #53389
• ddl,expression: introduce CtxWithHandleTruncateErrLevel to wrap a expression context to handle truncate error #53441
• ddl: introduce newDefaultReorgDistSQLCtx to replace mock context in DDL reorg #53467
• ddl: remove mock.Context usage in addIndexIngestWorker #53479
• ddl: split sessionctx.Context to sub contexts in CopContextBase #53642
• ddl: separate sessionctx.Context in backfillCtx to sub-contexts #53671
• ddl: remove useless fields in CopContextBase #53721