fix(get_mappings tool): fallback to raw JSON when mapping deserialization fails

[fatemehpourghaemi]

Summary

This PR improves the get_mappings tool by adding a fallback mechanism to return raw JSON when strict deserialization of Elasticsearch index mappings fails. Some indices have complex or unexpected mapping structures that don't match the existing Rust types, causing deserialization errors.

Changes

• Read the ES mapping response as raw JSON (serde_json::Value).
• Attempt strict deserialization into HashMap<String, Mappings>.
• On deserialization failure, return the raw JSON instead.
• This prevents errors and preserves tool functionality for all index mappings.

Testing

• Verified manually on indices with complex nested mappings.
• Existing end-to-end tests pass without issues.

Additional Context

Addresses an open issue: #173

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[smalenfant]

This has to be fixed - Would make it 100% more usable for Claude to figure out the mapping