refactor: propagate flush reasons through FlushRegions path

[QuakeWang]

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Refer to a related PR or issue link (optional)

Closes: #7586

What's changed and what's your intention?

This PR propagates flush reasons through the FlushRegions path, so metasrv-triggered flushes no longer fall back to Manual on the datanode and mito2 side.

It adds an optional reason to FlushRegions and RegionFlushRequest, propagates it through metasrv -> datanode -> mito2, and sets explicit reasons for region migration, repartition, and remote WAL prune flushes. Manual/user-triggered flush behavior remains unchanged, and legacy FlushRegions payloads without reason are still supported.

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.

[gemini-code-assist]

Code Review

This pull request introduces a reason field to region flush requests to categorize the source of flushes, such as region migration, repartitioning, or remote WAL pruning. The changes span multiple components, including common-meta, datanode, meta-srv, and the mito2 engine, ensuring that the flush reason is propagated from the instruction level down to the storage engine. The PR also includes updates to tests and simplifies RegionFlushRequest instantiation using default values. Feedback suggests a minor simplification for assigning the flush reason in the metadata service utility functions.

[WenyXu]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep them coming!

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[killme2008]

Good job!

@WenyXu Please take a look.

[WenyXu]

Rest LGTM