Add __repr__ methods to core RedisVL classes

Summary

Add useful __repr__ implementations for core RedisVL classes to improve debugging and developer ergonomics.

Level: Beginner

Current State

• Core objects currently use default object repr output (<Class object at 0x...>).
• This makes notebook debugging, test output, and agent-generated diagnostics hard to read.

Proposed Change

• Add concise, deterministic __repr__ methods to:
  • SearchIndex / AsyncSearchIndex in redisvl/index/index.py
  • SemanticCache in redisvl/extensions/cache/llm/semantic.py
  • SemanticRouter in redisvl/extensions/router/semantic.py
  • MessageHistory / SemanticMessageHistory in redisvl/extensions/message_history/
• Include only non-sensitive fields (name, prefix, storage type, counts).

Definition of Done

• Repr output is human-readable and stable.
• New unit tests verify expected repr strings.
• make format, make check-types, and tests pass.

Out of Scope

• Changing runtime behavior or API signatures.

Start with redisvl/index/index.py, redisvl/extensions/cache/llm/semantic.py, redisvl/extensions/router/semantic.py, and the files under redisvl/extensions/message_history/. Add stable, human-readable repr coverage for the named classes using only non-sensitive fields, then add unit tests for the expected strings. Run make format, make check-types, and the test suite to confirm the definition of done.