fix(ingester): clear pooled string slices before reuse

[xuezhichao]

This PR is for:

• Server

Fixes ingester pooled objects retaining old string references

Steps to reproduce the bug

• Ingest application logs, events, external metrics, or L7 flow logs with many native tag / metric string values.
• Release the objects back to their pools.
• The slices are reset with [:0], but their backing arrays still hold references to old strings.
• Heap memory may stay higher than expected because pooled objects keep those old strings reachable.

Changes to fix the bug

• Clear string slice elements before reusing pooled ApplicationLogStore.
• Clear string slice elements before reusing pooled EventStore.
• Clear string slice elements before reusing pooled ExtMetrics.
• Clear string slice elements before reusing pooled L7FlowLog.
• Keep slice capacity for reuse after clearing references.

Affected branches

• main

Checklist

This change only touches server-side ingester object reuse and does not affect eBPF programs.

• Added unit test to verify the fix.
• Verified eBPF program runs successfully on linux 4.14.x.
• Verified eBPF program runs successfully on linux 4.19.x.
• Verified eBPF program runs successfully on linux 5.2.x.