Fix/doc level monitor sample documents source fields

[carmeroa]

Description

This change fixes doc-level monitor sample_documents so action templates can access the original matched document source instead of percolation-transformed fields.

The implementation captures a deep copy of each matched document before field-name transformation for percolation and uses that data to populate sample_documents with a stable structure containing _id, _index, and _source.

This preserves original field names for Mustache templates, keeps nested objects intact, and avoids exposing percolation-mutated field names in action templates. It also reduces reliance on fallback mget for documents already captured during shard search, which is especially important for routed indices.

This PR also adds regression coverage for original source rendering, metadata exposure, nested object preservation, and the absence of percolation suffixes in sample_documents.

Related Issues

Resolves #2149

Check List

• New functionality includes testing.
  • See added regression tests in ./alerting/src/test/kotlin/org/opensearch/alerting/transport/DocLevelMonitorSampleDocumentsTests.kt.
• New functionality has been documented.
• API changes companion pull request created.
  • Not applicable: no API changes.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.
  • Documentation issue created: #12452.

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