[improvement](be) Apply trash sweep policy to shutdown tablets

[wenzhenghu]

Proposed changes

Issue Number: None

Related PR: HYDCP#75

Shutdown tablets are processed after the DataDir trash sweep. Previously, even when a manual cleanup or high disk usage made the current sweep delete trash immediately, newly resolved shutdown tablet paths could still be moved back into trash according to the global retention configuration. This delayed disk-space reclamation and recreated trash entries during an urgent cleanup.

This PR:

• builds one immutable sweep policy per DataDir and shares it between trash cleanup and shutdown-tablet resolution;
• deletes eligible shutdown tablet paths directly when trash retention is disabled, during manual cleanup, or above the existing high-watermark threshold;
• preserves move-to-trash behavior for normal retention sweeps;
• leaves shutdown tablets on unused DataDirs in the queue for a later sweep;
• keeps the existing transition, active-tablet, path, UID, TabletMeta, failure requeue, reference-count, and resolved-budget semantics;
• adds direct-delete result, timing, and reason metrics plus an unused-DataDir deferred gauge;
• adds BE unit coverage for the policy matrix, explicit filesystem modes, mixed DataDir policies, direct-delete failure retry, path-success/meta-failure retry, and intentional-skip metric semantics.

No FE-BE protocol, persistent format, or new configuration is introduced.

Release note

Shutdown tablet paths now follow the current DataDir trash sweep policy. During immediate cleanup conditions they are deleted directly instead of being moved into trash again.

Check List

• Formatting: ./build-support/clang-format.sh and ./build-support/check-format.sh passed
• Static checks: target-file clang-format 16 dry-run and git diff --check passed
• BE build: not completed; the local build was interrupted during third-party dependency preparation before source compilation
• BE unit tests: coverage added but not executed per the requested validation scope
• Regression tests: not run
• Behavior changed: Yes, only for shutdown tablets during immediate trash cleanup conditions
• Documentation required: No

[hello-stephen]

Thank you for your contribution to Apache Doris.
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