fix(mcp): replace TOCTOU race in Session.Initialize with sync.Once

[hannanmax]

Problem

Session.Initialize used a check-unlock-call-lock pattern for memoisation:

s.mu.Lock()
if s.initDone { /* return cached */ }
s.mu.Unlock()
// gap: another goroutine can enter here
res, raw, err := s.handshake(ctx)
s.mu.Lock()
s.initDone = true
// ...

Two goroutines that both observe initDone == false before the unlock can both enter handshake() concurrently. The second call's result overwrites the first's, and notifications/initialized can be sent twice — which violates the MCP spec and may confuse servers that track notification state.

Fix

Replace initDone bool with sync.Once. The Do block runs exactly once regardless of how many goroutines call Initialize simultaneously:

s.initOnce.Do(func() {
    res, raw, err := s.handshake(ctx)
    s.mu.Lock()
    s.initResult, s.initRaw, s.initErr = res, raw, err
    s.mu.Unlock()
    if err == nil {
        _ = s.notify(ctx, "notifications/initialized", nil)
    }
})
s.mu.Lock()
res, raw, err := s.initResult, s.initRaw, s.initErr
s.mu.Unlock()
return res, raw, err

Observable behaviour for sequential callers is identical. sync is already imported.