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

Summary

GoBGP 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`.
A 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.

Details

The issue is in the BGP OPEN capability parser under:

• pkg/packet/bgp/bgp.go
• pkg/packet/bgp/validate.go

The BGP OPEN optional parameter capability format includes a capability code, a capability length field, and a capability value. Each concrete capability decoder should only parse bytes inside the declared capability value boundary. In affected versions, the generic capability parser records the declared CapLen, but several concrete capability decoders continue parsing from the full remaining capability buffer after advancing past the two-byte capability header. Conceptually, the vulnerable pattern is:

data = data[2:]
// decoder reads from data without first limiting it to CapLen

### PoC
The following parser-level proof of concept demonstrates the issue without requiring a full BGP session or a running `bgpd` instance.
The malformed capability uses:
- Capability Code: `65` (`BGP_CAP_FOUR_OCTET_AS_NUMBER`)
- Declared `CapLen`: `0`
- Four following bytes: `00 00 fd e8`

Although the capability declares an empty value, affected versions parse the following four bytes as the 4-octet AS value `65000`.

### Impact
A 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.
In the 4-octet AS capability case, this may affect:
- peer AS validation;
- capability negotiation;
- interpretation of malformed OPEN messages;
- acceptance or rejection decisions during BGP session establishment.
This 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.