[Server] Fail loudly when a transport is used without Protocol::connect()

[chr-hertel]

Extracted slice from #9 (closed): fail loudly when a transport is used without Protocol::connect().

• BaseTransport's callback accessors now throw LogicException instead of silently returning []/null/doing nothing when the protocol callbacks were never wired (message handling, outgoing messages, pending requests, response finder, fiber yield).
• handleSessionEnd() stays tolerant: it is a teardown notification also reached via close() on error paths, where throwing would mask the original failure.
• Inlined the single-use ManagesTransportCallbacks trait into BaseTransport and dropped InMemoryTransport's redundant onMessage() override.

Confidence: 9/10 — small, fully covered by new unit tests; whole unit suite, cs-fixer and PHPStan green.