Add _WriteStream protocol for MicroPython Buffer write methods

[Copilot]

Summary

This PR implements reference stubs for the MicroPython Buffer protocol's extended write signature, as discussed in micropython/micropython#17938.

Changes

Added a new _WriteStream Protocol to reference/_mpy_shed/buffer_mp.pyi that defines three overloaded write method signatures:

# Basic write
stream.write(buf) -> int | None

# Write with maximum length
stream.write(buf, max_len) -> int | None

# Write with offset and maximum length  
stream.write(buf, off, max_len) -> int | None

This extended signature is used throughout MicroPython but was previously undocumented in the type stubs. It's commonly found in:

• Socket objects (socket.write)
• UART interfaces (machine.UART.write)
• I2C/SPI interfaces (machine.I2C.write, machine.SPI.write)
• Async stream objects (asyncio.Stream.write)

Design

• Uses Protocol (PEP 544) for structural subtyping, allowing any class with matching signatures to automatically conform without explicit inheritance
• Returns int | None to match MicroPython's behavior where non-blocking operations return None when they would block
• Leverages existing AnyReadableBuf type alias for buffer protocol compatibility
• Fully documented with docstrings for each signature variant

Backward Compatibility

No breaking changes. Existing AnyReadableBuf and AnyWritableBuf type aliases remain unchanged. The new protocol is exported from _mpy_shed for use in other modules.

Testing

• All modified .pyi files pass Python compilation
• Pyright type checking passes with 0 new errors
• Created standalone tests demonstrating protocol usage with socket-like and UART-like implementations

Closes #<issue_number>

Original prompt

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This section details on the original issue you should resolve

<issue_title>create mpy_shed/reference stubs for MicroPython Buffer protocol</issue_title>
<issue_description>
This is likely a good addition to reference_mpy_shed\buffer_mp.pyi
but care must be taken to verify correct operation and conflicts.

Also need to determine how to work with , or replace AnyReadableBuf and AnyWritableBuf

The signature is:

stream.write(buf)
stream.write(buf, max_len)
stream.write(buf, off, max_len)

I don't think this is actually documented anywhere, but it may be relied upon in some code.

Originally posted by @dpgeorge in micropython/micropython#17938 (comment)
</issue_description>

<agent_instructions>also chick if and how this can be used in other reference stubs.

changes MUST be restricted to the /reference folder </agent_instructions>

Comments on the Issue (you are @copilot in this section)

Fixes #833

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