[fix] Make clear operations idempotent

[0oshowero0]

Background

Previously, the clear operations (kv_clear, (async_)clear_samples, and (async_)clear_partition) were not idempotent: they would raise errors when asked to clear non-existent keys or partitions.

This strict behavior was originally intended to expose potential implementation bugs. As the codebase has matured, we now align these operations with the common idempotent clear semantics, where clearing non-existent data is a no-op and is only logged.

Changes

1. Controller-level clear_meta / clear_partition / reset_consumption

• Skip non-existent partitions and global indexes with an info/warning log instead of raising ValueError.
• In clear_meta, continue processing remaining partitions when one partition does not exist, instead of returning early.

2. kv_retrieve_meta

• When create=False, filter out non-existent keys and return a BatchMeta containing only the existing keys, instead of returning an empty BatchMeta.
• This makes downstream kv_clear naturally idempotent for partial or fully missing keys.

3. Logging

• Rephrase clear-related log messages to be grammatically correct and more natural (e.g., "Trying to clear a non-existent partition ... No action taken.").

4. Tests

• Add test_controller_clear_meta_idempotent to verify idempotent clear_meta behavior.
• Add test_controller_kv_retrieve_meta_partial_keys_without_create to verify partial key retrieval.
• Add test_kv_clear_idempotent in TestKVClearE2E to cover clearing non-existent keys, non-existent partitions, mixed existent/non-existent keys, and repeated clears.

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