GH-50901: [C++] Replace RapidJSON with simdjson in tensor extension types

[Reranko05]

Rationale for this change

This PR continues the simdjson migration by replacing RapidJSON usage in the FixedShapeTensorType and VariableShapeTensorType extension types with the existing simdjson helpers and On-Demand API.

Changes

• Replace RapidJSON parsing in FixedShapeTensorType::Deserialize with simdjson.
• Replace RapidJSON parsing in VariableShapeTensorType::Deserialize with simdjson.
• Use the existing simdjson helpers for JSON type and result handling.
• Remove the obsolete RapidJSON-specific JsonTypeName helper from the tensor extension utilities.
• Remove the remaining RapidJSON includes from the tensor extension utilities.
• Update deserialization tests to match simdjson JSON type names.

Fixes: #50901

• GitHub Issue: [C++] Replace RapidJSON with simdjson in tensor extension types #50901

[github-actions]

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