docs: document Redis JSON fallback and non-FastStream interop

[Lancetnik]

Description

Improves the Redis message-format documentation to explain how FastStream interoperates with non-FastStream publishers and subscribers, following the confusion raised in discussion #2899 after JSONMessageFormat / RedisJSONMessageParser was removed in 0.7.0.

What changed in docs/docs/en/redis/message_format.md:

• New "Interoperability with non-FastStream systems" section documenting that BinaryMessageFormatV1.parse already falls back to a legacy JSON envelope ({"data": ..., "headers": ...}) and then to raw bytes, so no dedicated JSON format is needed.
  • Consuming raw JSON from an external producer — a regular subscriber receives it as a dict with no extra config.
  • Publishing to an external consumer — either let the consumer parse the binary envelope (Go/Java examples), or send raw bytes via broker._connection (the native redis.asyncio.Redis client), with publish/rpush/xadd per destination type.
• New "Custom message format" section showing how to subclass MessageFormat.
• Removed stale JSONMessageFormat references from the switching examples and added a warning note about its removal.
• Updated the outdated "Migration plan" (which still claimed JSON was the current default) to a "Migration history" reflecting the 0.7.x state.

Fixes # (discussion #2899)

Type of change

• Documentation (typos, code examples, or any documentation updates)

Checklist

• My code adheres to the style guidelines of this project (just lint shows no errors)
• I have conducted a self-review of my own code
• I have made the necessary changes to the documentation
• My changes do not generate any new warnings
• I have added tests to validate the effectiveness of my fix or the functionality of my new feature
• Both new and existing unit tests pass successfully on my local environment by running just test-coverage
• I have ensured that static analysis tests are passing by running just static-analysis
• I have included code examples to illustrate the modifications