feat(lsp): capture server stderr to diagnose init failures

drainStderr discarded all server stderr, so an LSP init failure (e.g. 'command not found', missing module) surfaced as an opaque 'initialization failed'. Retain a bounded tail (StderrCaptureMax=4096B, oldest dropped) under a mutex and attach it to the init-failure error context. Adds TestClientStderrCapture.

internal/lsp/client_test.go

@@ -0,0 +1,28 @@
+package lsp
+
+import (
+	"strings"
+	"testing"
+)
+
+// drainStderr retains a bounded tail of server stderr for diagnostics; capture
+// must keep recent bytes and never grow past StderrCaptureMax.
+func TestClientStderrCapture(t *testing.T) {
+	c := &Client{}
+
+	if c.RecentStderr() != "" {
+		t.Errorf("expected empty stderr initially, got %q", c.RecentStderr())
+	}
+
+	c.captureStderr([]byte("gopls: command not found\n"))
+	if !strings.Contains(c.RecentStderr(), "command not found") {
+		t.Errorf("expected captured stderr, got %q", c.RecentStderr())
+	}
+
+	// Exceed the cap: only the most recent StderrCaptureMax bytes are kept.
+	big := strings.Repeat("x", StderrCaptureMax*2)
+	c.captureStderr([]byte(big))
+	if got := len(c.RecentStderr()); got > StderrCaptureMax {
+		t.Errorf("stderr tail not bounded: len=%d, want <= %d", got, StderrCaptureMax)
+	}
+}