ext/snmp: promote invalid-input warnings to ValueError

[prateekbhujel]

Fixes#21318 by promoting remaining invalid-input warning paths in ext/snmp to ValueError for consistency with PHP 8+ argument validation behavior.

Changed paths:

• Malformed IPv6 host (snmp_session_init)
• Missing type/value in OID arrays (php_snmp_parse_oid)
• Invalid contextEngineID (snmp_session_set_contextEngineID)

Tests updated accordingly:

• ext/snmp/tests/ipv6.phpt
• ext/snmp/tests/snmpset.phpt
• ext/snmp/tests/snmp2_set.phpt
• ext/snmp/tests/snmp-object-setSecurity_error.phpt

[iluuu1994]

Hi @prateekbhujel! Thanks for your contribution. Per new policy, promotions of errors are considered breaking and require a discussion on the internals mailing list and frequently an RFC.

[prateekbhujel]

@iluuu1994 Thanks for clarifying the new policy. I will start an internals thread for this BC change and link it here before moving this PR forward.

[prateekbhujel]

@iluuu1994 Thanks again for the guidance. I started the internals discussion here: https://news-web.php.net/php.internals/130231

[prateekbhujel]

@iluuu1994 Quick update — I noticed the related policy RFC has since been published: https://wiki.php.net/rfc/policy-exempt-type-value-error-bc-policy. I'm waiting for that vote to conclude before proceeding. Happy to move forward once the policy direction is clear.

[prateekbhujel]

@iluuu1994 The policy RFC has been accepted, so I think this PR fits the current BC policy now. Could you re-review when you get a chance?
RFC: https://wiki.php.net/rfc/policy-exempt-type-value-error-bc-policy

[Girgias]

I think the only final remark I have would be to add UNEXPECTED() compiler hints for the exception code paths. :)

[Girgias]

I'll wait for CI and merge afterwards :)

[prateekbhujel]

@Girgias Following up on your readability note from #21319, I opened #21723 as a small rename-only cleanup for the argument-number variables.