feat(signal): support POSIX-compliant SIGCONT and SIGSTOP handling

[TomGoh]

Changes:

• check_signals_slow would return Option<(SignalInfo, Option<SignalOSAction>)> instead of Option<(SignalInfo, SignalOSAction)> to allow reporting signals without OS actions (e.g., ignored SIGCONT, which should be handled as normal when the target process is stopped).
• Add special handling path in check_signals_slow() to return ignored SIGCONT with a None action, enabling side effects without handler execution.
• Add signal management utilities: remove_signal(), has_signal(), flush_stop_signals() for process/thread signal managers.
• Implement PendingSignals::remove_signal() and has_signal() for direct queue manipulation in support for upper process/thread layer operations.
• Add SignalEventFlags bitflag struct to save a pending stop/continue event of a process.
• Add signal_events and last_stop_signal field in ProcessSignalManager to save stoppage due to signals for a process. When a process is stopped or continued due to signals, the corresponding event would be set in the signal_event flag and the stopping signal would be saved in last_stop_signal.
• Add set_stop_signal, set_cont_signal, peek_pending_stop_event, peek_pending_cont_event, consume_stop_event, and consume_cont_event for ProcessSignalManager. The stop/continue event would be set and the signal would be saved in the set_x_signal function. peek_pending_x_event would only examine whether there is a pending event and the saved signal, but does not make any modifications. consume_x_event would clear the saved signal and clear the signal_event flag.