[BEAM] Fix Python VarIntCoder OverflowError on uint64 values

[AviKndr]

What

VarIntCoder raises OverflowError when handed a Python int in the unsigned 64-bit range [2**63, 2**64) (a uint64), even though such a value has a well-defined VarInt encoding.

Why

In the Cython build, the stream methods take a signed int64_t:

cpdef write_var_int64(self, libc.stdint.int64_t signed_v):
    cdef libc.stdint.uint64_t v = signed_v   # body already works on unsigned

The body re-casts to uint64_t and encodes the bit pattern correctly, but Cython converts the incoming Python int to the signed int64_t parameter at the call boundary — before the body runs — so any uint64 value is rejected with OverflowError. The same applies to get_varint_size used by estimate_size.

Fix

A uint64 value v and the signed int64 v - 2**64 (its two's-complement twin) produce identical VarInt bytes. VarIntCoderImpl now folds uint64 values to that signed twin before they cross into Cython, in both encode_to_stream and estimate_size:

def _as_signed_int64(value):
  if (1 << 63) <= value < (1 << 64):
    return int(value) - (1 << 64)
  return value

• Wire-compatible — byte-identical to Java's signed VarIntCoder.
• No regression — normal signed ints pass through untouched; the pure-Python slow_stream path produces the same bytes.
• Still bounded — values >= 2**64 are left unchanged and still overflow in the Cython path, preserving the coder's 64-bit contract.

Note: decoding remains signed (read_var_int64 returns int64_t), matching Java — so the encoding of 2**64 - 1 decodes back to -1. This change removes the crash and guarantees correct wire bytes; it does not make the coder round-trip uint64 back to unsigned (that would be a separate coder).

Tests

Adds test_varint_coder_uint64 to the shared coders_test_common.py suite (runs both compiled and uncompiled): no-overflow encoding, wire equivalence to the signed twin, size estimation, signed decode semantics, and the still-raising out-of-range case (guarded on the compiled implementation).

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