feat: expand InitializationState with explicit lifecycle state machine

[dgenio]

Summary

Addresses #1691 — Phase 1: expand the InitializationState enum with an explicit lifecycle state machine and add centralized transition validation.

Problem

The current ServerSession lifecycle management uses a simple three-state enum (NotInitialized, Initializing, Initialized) with direct state assignment. This makes it difficult to reason about session lifecycle, provides no protection against invalid transitions, and lacks visibility into shutdown states. Stateless sessions were handled by jumping directly to Initialized, conflating two conceptually different modes.

Solution

Expanded InitializationState enum

• Stateless — sessions that skip the initialization handshake (previously mapped to Initialized)
• Closing — session is shutting down, entered during __aexit__
• Closed — terminal state after shutdown completes
• Error — unrecoverable failure, with recovery paths to Closing/Closed

Centralized transition validation

• _VALID_TRANSITIONS dict — maps each state to its valid target states, documenting the state machine in one place
• _transition_state() method — validates transitions and raises RuntimeError on invalid ones, with debug logging
• Re-initialization (Initialized → Initializing) is explicitly allowed for backward compatibility

New properties

• initialization_state — read-only access to current state
• is_initialized — returns True for both Initialized and Stateless (replaces direct comparisons)

__aexit__ override

• Transitions through Closing → Closed on session exit
• Gracefully handles edge cases (already closing, error states)

Updated handlers

• _received_request and _received_notification now use _transition_state() instead of direct assignment
• _received_notification guards against redundant InitializedNotification in stateless mode

Tests

20 new tests in tests/server/test_session_lifecycle.py covering:

• Enum completeness and value uniqueness
• Transition table correctness (all states have entries, Closed is terminal, Error allows recovery)
• Valid and invalid state transitions via _transition_state()
• is_initialized property for all states
• __aexit__ lifecycle transitions
• Full client/server handshake integration test

All 474 existing server tests pass with no regressions.