[php-src] Issue #21318: ext/snmp: Promote 4 E_WARNING to ValueError for invalid input

Issue: https://github.com/php/php-src/issues/21318
Author: thomasvincent

## Description

Four validation checks in ext/snmp still emit `E_WARNING` and return `false` for invalid input arguments, rather than throwing `ValueError` as is standard practice since PHP 8.0.

## Affected locations

1. **Malformed IPv6 address** (`snmp_session_init`) — hostname like `[dead:beef::` (missing closing `]`) emits `E_WARNING` instead of throwing `ValueError`.

2. **Type array too short** (`php_snmp_parse_oid`) — when `snmpset()` receives fewer types than OIDs, emits `E_WARNING("no type set")` instead of `ValueError`.

3. **Value array too short** (`php_snmp_parse_oid`) — same pattern as #2 for values.

4. **Invalid context engine ID** (`snmp_session_set_contextEngineID`) — non-hex string emits `E_WARNING`. The existing code already has a `// TODO Promote to Error?` comment requesting this change.

## Notes

- The IPv6 validation is called from both the procedural API (hostname = arg 1) and the OO `SNMP::__construct` (hostname = arg 2). A plain `zend_value_error()` is appropriate rather than `zend_argument_value_error()` to avoid showing a wrong argument position.
- The `generate_Ku()` warnings (authentication/privacy key generation, lines ~1089/1105) are intentionally left as `E_WARNING` since those can fail for runtime reasons (library configuration, unsupported algorithm), not purely bad input.
- This is a backward compatibility break: code relying on `set_error_handler()` + checking for `false` returns will now get uncaught `ValueError` exceptions.

## PHP Version

master (targeting next minor/major)