Simplify RequestMessage class hierarchy

[jyemin]

• Remove EncodingMetadata abstraction and pull the remains into CommandMessage
• Pull the writeDocuments method into CommandMessage and simplify its parameters

JAVA-5944

[Copilot]

Pull Request Overview

This PR simplifies the RequestMessage class hierarchy by removing the EncodingMetadata abstraction layer and consolidating its functionality directly into CommandMessage. The changes eliminate unnecessary indirection and streamline the encoding process for MongoDB Wire Protocol messages.

Key Changes:

• Remove the EncodingMetadata class and its associated methods from RequestMessage
• Simplify the encoding method signature by removing metadata return value
• Move document writing logic directly into CommandMessage with a more specific implementation

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
RequestMessage.java	Removes EncodingMetadata class and simplifies encoding method signature
CompressedMessage.java	Updates encoding method to match new signature without metadata return
CommandMessage.java	Adds direct tracking of first document position and includes document writing logic

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