test: add concurrent publish/subscribe stress coverage for membus (#167)

[amankoli09]

Closes #167

📌 Description

This PR adds full thread-safe publish/subscribe capabilities to the in-memory bus implementation in

internal/bus/membus/membus.go
and introduces an extensive concurrent publish/subscribe stress test suite in

internal/bus/membus/membus_test.go
to ensure parity with natsbus under high concurrency.

🧩 Changes Implemented

Pub/Sub Parity in membus.go:

Added thread-safe Subscribe(subject string, handler func(Message)) (*Subscription, error) and Unsubscribe() error methods.
Designed fine-grained, deadlock-free synchronization where bus mutexes (b.mu) and subscription mutexes (s.mu) are never acquired out of order.
Subscriber handlers are invoked outside b.mu.Lock(), preventing deadlocks and simulating NATS decoupling where message publishing does not block on consumer callback execution.
Every subscriber handler receives a defensive copy of message payloads (Message.Data) to prevent concurrent slice mutations across overlapping subscribers.
Concurrency Stress Tests & Edge Case Coverage in membus_test.go:

TestConcurrentPublishSubscribe_NoMessageLoss: Spins up $N$ concurrent publishers and $M$ subscribers on shared topics; asserts exact message delivery counts across overlapping subscribers with zero dropped messages.
TestConcurrentPublishSubscribe_StressAndUnsubscribe: Stress test with 20 concurrent publishers and 15 concurrent subscribers repeatedly subscribing, receiving messages, and unsubscribing while publishes are actively in flight.
TestSubscriberUnsubscribeMidPublish: Explicitly verifies that calling sub.Unsubscribe() from within an active message callback mid-publish does not panic, deadlock, or affect other subscribers on the same topic.
TestPublishZeroSubscribers: Asserts that publishing to topics with zero subscribers correctly captures messages in bus history (b.Messages()) without errors or leaks.
TestManyTopicsOverlappingSubscribers: Validates exact message delivery across multiple overlapping subscribers listening to shared topics.
TestSubscriptionEdgeCases: Validates error handling (nil handler rejection, subscribing after bus Close(), zero-value bus initialization, and idempotent Unsubscribe()/Close() calls).