[Fix] Warn on task_timeout less than symcache symbol timeout

[Fix] Warn on task_timeout less than symcache symbol timeout

Call rspamd_worker_check_and_adjust_timeout during configtest so
misconfigured plugin timeouts are reported at configuration validation
time. Elevate the diagnostic from info to warning level.

Fix rspamd_symcache_add_symbol_augmentation to parse "key=value" format
in augmentation strings, allowing numeric timeout augmentations from
Lua plugins to be stored and compared correctly.

Clarify mx_check timeout comment to explain that the effective symbol
timeout includes dns.timeout in addition to the configured value.