chore: Refactor ESDK message-body.md

[lucasmcdonald3]

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Description of changes:

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Pull request overview

Refactors the ESDK data format documentation for the message body to make serialization requirements more explicit and align prose with normative language.

Changes:

• Removes the top-level “Version”, “Implementations”, and RFC2119 conventions blocks from message-body.md.
• Adds explicit MUST-level serialization order statements for non-framed data, regular frames, and final frames.
• Adds additional MUST-level requirements for field sizes and interpretations (bytes vs integer types).

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