Cache OpenSearch version to avoid redundant HTTP calls per tool invocation

[ShivaniNR]

Description

Cache the OpenSearch cluster version after the first successful retrieval to eliminate the redundant HTTP call made by get_opensearch_version() on every tool invocation. Previously, each tool call made 2 HTTP round-trips (version check + actual operation); now, subsequent calls return the cached version.

• Add _version_cache module-level dict in helper.py keyed by cluster name
• Add clear_version_cache() for cache reset
• Add autouse conftest fixture to prevent cache leaking between tests
• Add 4 tests covering cache hit, per-cluster isolation, failure retry, and cache clearing

Issues Resolved

Closes #185

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