GoBGP: BGP OPEN capability parser may read capability values outside declared CapLen boundaries

 1GoBGP contains a BGP OPEN capability parsing issue where several concrete capability decoders may parse data from the full remaining capability buffer instead of the slice bounded by the declared capability length, `CapLen`.
 2A malformed BGP OPEN message can cause bytes from a following capability to be interpreted as part of the current capability. The most security-relevant case is the 4-octet AS capability, where a capability with `CapLen == 0` may cause the parser to read bytes from the following capability as the 4-octet AS value. This parsed value may later affect peer AS validation during BGP session establishment.

 1data = data[2:]
 2// decoder reads from data without first limiting it to CapLen
 3 
 4### PoC
 5The following parser-level proof of concept demonstrates the issue without requiring a full BGP session or a running `bgpd` instance.
 6The malformed capability uses:
 7- Capability Code: `65` (`BGP_CAP_FOUR_OCTET_AS_NUMBER`)
 8- Declared `CapLen`: `0`
 9- Four following bytes: `00 00 fd e8`
10 
11Although the capability declares an empty value, affected versions parse the following four bytes as the 4-octet AS value `65000`.
12 
13### Impact
14A remote peer that can send a malformed BGP OPEN message to a GoBGP instance may cause capability values to be parsed from outside their declared `CapLen` boundaries.
15In the 4-octet AS capability case, this may affect:
16- peer AS validation;
17- capability negotiation;
18- interpretation of malformed OPEN messages;
19- acceptance or rejection decisions during BGP session establishment.
20This issue does not appear to be arbitrary memory corruption, remote code execution, or information disclosure. It is a protocol parser boundary validation issue that can affect BGP OPEN validation semantics.