spotbugs(WEM) Batch 3: enrich Session IllegalStateException messages (5 cleared)

Adds in-scope state (slotId + transcript size) to the 5 state-machine
IllegalStateException messages thrown by Session.send / stream /
commitStreamedReply / save / restore.

Before:
    throw new IllegalStateException(
        "stream in progress; call commitStreamedReply(...) before send(...)");

After:
    throw new IllegalStateException(
        "stream in progress on slot " + slotId
            + " (transcript=" + turns.size() + " turns)"
            + "; call commitStreamedReply(...) before send(...)");

Why both fields:
- slotId identifies which session got stuck in a multi-session setup
  (a process can hold many Sessions on the same model, one per slot).
- turns.size() shows how far the transcript progressed before the
  state-machine violation — useful when triaging "the session went
  weird around message N" reports.

Compatibility:
- Session is final (line 41) — no subclass override risk.
- No test asserts on the exception message text (verified across
  SessionConcurrencyTest, LlamaModelTest).

THROWS_METHOD_THROWS_RUNTIMEEXCEPTION findings on Session.send and
Session.stream (the catch+cleanup+rethrow pattern at line 112 / 138)
are deliberately NOT addressed in this batch; they are deferred to
their own investigation alongside the existing BAF suppression
(spotbugs/spotbugs#3918 + PR #4087 lifecycle).

SpotBugs Max+Low: WEM goes 10 -> 5. Total jllama: 39 -> 34.

Author: claude

src/main/java/net/ladenthin/llama/Session.java

@@ -99,7 +99,10 @@ public Session(
     public String send(String userMessage) {
         synchronized (lock) {
             if (streamingActive) {
-                throw new IllegalStateException("stream in progress; call commitStreamedReply(...) before send(...)");
+                throw new IllegalStateException(
+                        "stream in progress on slot " + slotId
+                                + " (transcript=" + turns.size() + " turns)"
+                                + "; call commitStreamedReply(...) before send(...)");
             }
             turns.add(new Pair<String, String>("user", userMessage));
             InferenceParameters params = buildParams();
@@ -126,7 +129,10 @@ public String send(String userMessage) {
     public LlamaIterable stream(String userMessage) {
         synchronized (lock) {
             if (streamingActive) {
-                throw new IllegalStateException("stream in progress; call commitStreamedReply(...) before stream(...)");
+                throw new IllegalStateException(
+                        "stream in progress on slot " + slotId
+                                + " (transcript=" + turns.size() + " turns)"
+                                + "; call commitStreamedReply(...) before stream(...)");
             }
             turns.add(new Pair<String, String>("user", userMessage));
             try {
@@ -149,7 +155,10 @@ public LlamaIterable stream(String userMessage) {
     public void commitStreamedReply(String assistantText) {
         synchronized (lock) {
             if (!streamingActive) {
-                throw new IllegalStateException("no stream in progress; call stream(...) first");
+                throw new IllegalStateException(
+                        "no stream in progress on slot " + slotId
+                                + " (transcript=" + turns.size() + " turns)"
+                                + "; call stream(...) first");
             }
             turns.add(new Pair<String, String>("assistant", assistantText));
             streamingActive = false;
@@ -165,7 +174,10 @@ public void commitStreamedReply(String assistantText) {
     public String save(String filepath) {
         synchronized (lock) {
             if (streamingActive) {
-                throw new IllegalStateException("stream in progress; call commitStreamedReply(...) before save(...)");
+                throw new IllegalStateException(
+                        "stream in progress on slot " + slotId
+                                + " (transcript=" + turns.size() + " turns)"
+                                + "; call commitStreamedReply(...) before save(...)");
             }
             return model.saveSlot(slotId, filepath);
         }
@@ -181,7 +193,9 @@ public String restore(String filepath) {
         synchronized (lock) {
             if (streamingActive) {
                 throw new IllegalStateException(
-                        "stream in progress; call commitStreamedReply(...) before restore(...)");
+                        "stream in progress on slot " + slotId
+                                + " (transcript=" + turns.size() + " turns)"
+                                + "; call commitStreamedReply(...) before restore(...)");
             }
             return model.restoreSlot(slotId, filepath);
         }