ProtoSize::calculate_size() already computes the exact encoded size
before encode() runs and is the boundary validation. The buffer is
pre-sized to match. Since the buffer size is always correct,
push_back() capacity checks on every byte are redundant overhead.
Rename ProtoWriteBuffer to ProtoWritePreSizedBuffer to document the
contract. Write through a raw uint8_t* pointer instead of push_back().
Pre-resize the buffer to include payload space before encoding.
Add ESPHOME_DEBUG_API bounds checks to validate writes stay within the
pre-sized region during development and integration testing.
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