refactor: refactor instruction handler and adds support for batch region downgrade operations

[WenyXu]

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Refer to a related PR or issue link (optional)

#7021

What's changed and what's your intention?

This PR refactors the instruction handler of datanode and adds support for batch region downgrade operations, as preparatory work for batch region migration.

Key Changes

• Renamed Instruction::DowngradeRegion to Instruction::DowngradeRegions with Vec<DowngradeRegion> support
• Renamed InstructionReply::DowngradeRegion to InstructionReply::DowngradeRegions with Vec<DowngradeRegionReply> return type
• Added region_id field to DowngradeRegionReply for identifying individual responses in batch operations
• Implemented parallel processing of multiple region downgrades using join_all
• Updated all related tests and mock utilities to work with the new batch format

PR Checklist

Please convert it to a draft if some of the following conditions are not met.

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.
• API changes are backward compatible.
• Schema or data changes are backward compatible.

[fengjiachun]

Rest LGTM