fix(varint): reject 10-byte terminator overflow and align error types

[acul71]

Fixes #1458

What was wrong?

A 10-byte uvarint encoding whose terminating byte still carries extra payload bits could decode to a value greater than 2^64-1 (e.g. decode_uvarint(b"\xff" * 9 + b"\x7f")). Over-long buffer-path failures also raised ValueError while the stream decoder raised ParseError, so callers catching only one type handled the two malformation classes inconsistently.

How was it fixed?

• Added a shared _reject_uvarint_overflow guard used by decode_uvarint and decode_uvarint_from_stream before returning a decoded value.
• Unified over-long buffer-path failures to ParseError with the same message as the stream decoder.
• Expanded docstrings for decode_uvarint_from_stream and decode_varint_with_size (truncation/partial-return contract documented; behavior of decode_varint_with_size unchanged for WebRTC except ValueError guards).
• Added rejection tests for 11-byte encodings and 10-byte terminator overflow on both buffer and stream paths.

Test plan

• pytest tests/core/utils/test_varint.py tests/core/utils/test_varint_message_limits.py
• make lint
• make typecheck
• make test
• make linux-docs

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