…-ai#1164 agentscope-ai#1165 agentscope-ai#1166) (agentscope-ai#1167)

## Summary

This PR fixes three bugs identified in the core library:

- **agentscope-ai#1164** \`StaticLongTermMemoryHook\` injects retrieved memory at the
beginning of the message list with a \`SYSTEM\` role, which breaks
message sequence constraints and degrades RAG quality
- **agentscope-ai#1165** \`ToolGroupManager.activeGroups\` uses a non-thread-safe
\`ArrayList\`, causing race conditions in concurrent agent scenarios
- **agentscope-ai#1166** \`MessageUtils.extractRecentToolCalls\` calls \`.equals()\`
on \`msg.getName()\` without a null check, throwing
\`NullPointerException\` when an assistant message has no name

## Changes

### Fix agentscope-ai#1164 — \`StaticLongTermMemoryHook.java\`

Change memory message role from \`SYSTEM\` to \`USER\` and append it at
the end of the message list so the sequence remains valid and the memory
context is closest to the model's generation point:

\`\`\`java
// Before
Msg memoryMsg = Msg.builder().role(MsgRole.SYSTEM)...build();
List<Msg> enhancedMessages = new ArrayList<>();
enhancedMessages.addAll(inputMessages);
enhancedMessages.add(memoryMsg);   // appended at end but wrong role

// After
Msg memoryMsg = Msg.builder().role(MsgRole.USER)...build();
List<Msg> enhancedMessages = new ArrayList<>(inputMessages);
enhancedMessages.add(memoryMsg);   // USER role, appended at end
\`\`\`

### Fix agentscope-ai#1165 — \`ToolGroupManager.java\`

Replace the non-thread-safe \`ArrayList\` with \`CopyOnWriteArrayList\`
(declared \`volatile\`) and update \`setActiveGroups()\` / \`copyTo()\`
to replace the reference atomically, avoiding the transient empty-list
window that \`clear()\` + \`addAll()\` would create:

\`\`\`java
// Before
private List<String> activeGroups = new ArrayList<>();
// ...
this.activeGroups = new ArrayList<>(activeGroups);
target.activeGroups = new ArrayList<>(this.activeGroups);

// After
private volatile List<String> activeGroups = new
CopyOnWriteArrayList<>();
// ...
this.activeGroups = new CopyOnWriteArrayList<>(activeGroups);
target.activeGroups = new CopyOnWriteArrayList<>(this.activeGroups);
\`\`\`

### Fix agentscope-ai#1166 — \`MessageUtils.java\`

Replace \`msg.getName().equals(agentName)\` with \`Objects.equals()\` to
safely handle \`null\` names:

\`\`\`java
// Before
if (msg.getRole() == MsgRole.ASSISTANT &&
msg.getName().equals(agentName)) {

// After
if (msg.getRole() == MsgRole.ASSISTANT && Objects.equals(msg.getName(),
agentName)) {
\`\`\`

## Test plan

- [x] Existing unit tests pass (\`mvn test -pl agentscope-core\`)
- [x] \`StaticLongTermMemoryHookTest\` updated to assert \`USER\` role
and end-of-list position
- [ ] Verify no \`ConcurrentModificationException\` when multiple agents
modify tool groups concurrently
- [ ] Verify no \`NullPointerException\` when assistant messages without
names are present in memory