test: add regression tests for #45 hang fixes

- MCP server: 4 tests verifying concurrent dispatch, slow-request
  isolation, semaphore limiting, and graceful drain on cancel
- Embedding: 4 tests verifying context-aware mutex cancellation,
  uncontended success, batch cancellation, and cleanup after cancel
- Vector client: 3 tests for acquireRLockWithContext cancel, success,
  and cleanup goroutine correctness
- Worker handlers: 1 test verifying handleSearchByPrompt inherits
  request context cancellation (skips without FTS5)

12 regression tests total covering the four fix areas.

internal/embedding/service_test.go

@@ -1,9 +1,12 @@
 package embedding
 
 import (
+	"context"
+	"errors"
 	"math"
 	"sync"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -310,6 +313,146 @@ func TestEmbed_Deterministic(t *testing.T) {
 	}
 }
 
+// --- Regression tests: context-aware mutex (Fix #45) ---
+
+// asBGE casts the service model to *bgeModel for direct mutex access.
+// Tests are in the same package so this is safe.
+func asBGE(t *testing.T, svc *Service) *bgeModel {
+	t.Helper()
+	m, ok := svc.model.(*bgeModel)
+	require.True(t, ok, "model is not *bgeModel — test invariant broken")
+	return m
+}
+
+// holdMutex locks m.mu in a background goroutine and returns a release func.
+// The returned ready channel is closed once the lock is held.
+func holdMutex(m *bgeModel) (ready <-chan struct{}, release func()) {
+	ch := make(chan struct{})
+	done := make(chan struct{})
+	go func() {
+		m.mu.Lock()
+		close(ch)   // signal: lock acquired
+		<-done      // wait for release signal
+		m.mu.Unlock()
+	}()
+	return ch, func() { close(done) }
+}
+
+// TestEmbedWithContext_CancelWhileWaitingForMutex is the core regression test.
+// If the mutex is held and the context times out, EmbedWithContext must return
+// immediately with a context error — not block until the mutex is released.
+func TestEmbedWithContext_CancelWhileWaitingForMutex(t *testing.T) {
+	svc, err := NewService()
+	require.NoError(t, err)
+	defer svc.Close()
+
+	m := asBGE(t, svc)
+
+	// Hold the mutex to simulate a stuck ONNX call.
+	ready, release := holdMutex(m)
+	<-ready // ensure lock is held before proceeding
+	defer release()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 100*time.Millisecond)
+	defer cancel()
+
+	start := time.Now()
+	_, err = svc.EmbedWithContext(ctx, "test text")
+	elapsed := time.Since(start)
+
+	// Must return a context error.
+	require.Error(t, err)
+	assert.True(t,
+		errors.Is(err, context.DeadlineExceeded) || errors.Is(err, context.Canceled),
+		"expected context error, got: %v", err)
+
+	// Must return quickly (well under the 30 s default; allow 2× the timeout for CI slack).
+	assert.Less(t, elapsed, 200*time.Millisecond,
+		"EmbedWithContext blocked too long: %v", elapsed)
+}
+
+// TestEmbedWithContext_SuccessWhenUncontended verifies normal operation still works.
+func TestEmbedWithContext_SuccessWhenUncontended(t *testing.T) {
+	svc, err := NewService()
+	require.NoError(t, err)
+	defer svc.Close()
+
+	emb, err := svc.EmbedWithContext(context.Background(), "hello world")
+	require.NoError(t, err)
+
+	assert.Len(t, emb, EmbeddingDim)
+
+	var sum float32
+	for _, v := range emb {
+		sum += v * v
+	}
+	assert.Greater(t, sum, float32(0), "embedding should not be all zeros")
+}
+
+// TestEmbedBatchWithContext_CancelDuringBatch verifies batch embedding respects
+// context cancellation while blocked on mutex acquisition.
+func TestEmbedBatchWithContext_CancelDuringBatch(t *testing.T) {
+	svc, err := NewService()
+	require.NoError(t, err)
+	defer svc.Close()
+
+	m := asBGE(t, svc)
+
+	ready, release := holdMutex(m)
+	<-ready
+	defer release()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 100*time.Millisecond)
+	defer cancel()
+
+	start := time.Now()
+	_, err = svc.EmbedBatchWithContext(ctx, []string{"a", "b", "c"})
+	elapsed := time.Since(start)
+
+	require.Error(t, err)
+	assert.True(t,
+		errors.Is(err, context.DeadlineExceeded) || errors.Is(err, context.Canceled),
+		"expected context error, got: %v", err)
+	assert.Less(t, elapsed, 200*time.Millisecond,
+		"EmbedBatchWithContext blocked too long: %v", elapsed)
+}
+
+// TestEmbedWithContext_CleanupAfterCancel verifies the cleanup goroutine in
+// acquireMutex properly unlocks the mutex after context cancellation,
+// so subsequent calls do not deadlock.
+func TestEmbedWithContext_CleanupAfterCancel(t *testing.T) {
+	svc, err := NewService()
+	require.NoError(t, err)
+	defer svc.Close()
+
+	m := asBGE(t, svc)
+
+	// --- first call: context expires while mutex is held ---
+	ready, release := holdMutex(m)
+	<-ready
+
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Millisecond)
+	defer cancel()
+
+	_, firstErr := svc.EmbedWithContext(ctx, "should fail")
+	require.Error(t, firstErr)
+	assert.True(t,
+		errors.Is(firstErr, context.DeadlineExceeded) || errors.Is(firstErr, context.Canceled),
+		"expected context error on first call, got: %v", firstErr)
+
+	// Release the held mutex so the cleanup goroutine inside acquireMutex can finish.
+	release()
+
+	// Give the cleanup goroutine a moment to acquire-and-release the mutex.
+	// 50 ms is generous; the goroutine only has to lock+unlock with no contention.
+	time.Sleep(50 * time.Millisecond)
+
+	// --- second call: mutex should be free, no deadlock ---
+	emb, secondErr := svc.EmbedWithContext(context.Background(), "should work")
+	require.NoError(t, secondErr, "second call should succeed after cleanup goroutine released mutex")
+	assert.Len(t, emb, EmbeddingDim)
+}
+
 // Helper function to calculate cosine similarity
 func cosineSimilarity(a, b []float32) float64 {
 	if len(a) != len(b) {