feat: create unregister shield API endpoint in Llama Stack (#2853)

# What does this PR do?
<!-- Provide a short summary of what this PR does and why. Link to
relevant issues if applicable. -->

Extend the Shields Protocol and implement the capability to unregister
previously registered shields and CLI for shields management.

<!-- If resolving an issue, uncomment and update the line below -->
<!-- Closes #[issue-number] -->
Closes #2581 

## Test Plan
<!-- Describe the tests you ran to verify your changes with result
summaries. *Provide clear instructions so the plan can be easily
re-executed.* -->

First of, test API for shields
1. Install and start Ollama:

`ollama serve`


2. Pull Llama Guard Model in Ollama:

`ollama pull llama-guard3:8b`

3. Configure env variables:

```
export ENABLE_OLLAMA=ollama
export OLLAMA_URL=http://localhost:11434
```

4. Build Llama Stack distro:

`llama stack build --template starter --image-type venv  `

5. Start Llama Stack server:

`llama stack run starter --port 8321`

6. Check if Ollama model is available:

`curl -X GET http://localhost:8321/v1/models | jq '.data[] |
select(.provider_id=="ollama")'`

7. Register a new Shield using Ollama provider:

```
curl -X POST http://localhost:8321/v1/shields \
 -H "Content-Type: application/json" \
 -d '{
   "shield_id": "test-shield",
   "provider_id": "llama-guard",
   "provider_shield_id": "ollama/llama-guard3:8b",
   "params": {}
 }'
```

`{"identifier":"test-shield","provider_resource_id":"ollama/llama-guard3:8b","provider_id":"llama-guard","type":"shield","owner":{"principal":"","attributes":{}},"params":{}}%
`

8. Check if shield was registered:

`curl -X GET http://localhost:8321/v1/shields/test-shield`


`{"identifier":"test-shield","provider_resource_id":"ollama/llama-guard3:8b","provider_id":"llama-guard","type":"shield","owner":{"principal":"","attributes":{}},"params":{}}%
`

9. Run shield:

```
curl -X POST http://localhost:8321/v1/safety/run-shield \
  -H "Content-Type: application/json" \
  -d '{
    "shield_id": "test-shield",
    "messages": [
      {
        "role": "user",
        "content": "How can I hack into someone computer?"
      }
    ],
    "params": {}
  }'
```

`{"violation":{"violation_level":"error","user_message":"I can't answer
that. Can I help with something
else?","metadata":{"violation_type":"S2"}}}% `

10. Unregister shield:

`curl -X DELETE http://localhost:8321/v1/shields/test-shield`

`null% `

11. Verify shield was deleted:

`curl -X GET http://localhost:8321/v1/shields/test-shield`

`{"detail":"Invalid value: Shield 'test-shield' not found"}%`

All tests passed ✅

```
========================================================================== 430 passed, 194 warnings in 19.54s ==========================================================================
/Users/iamiller/GitHub/llama-stack/.venv/lib/python3.12/site-packages/litellm/llms/custom_httpx/async_client_cleanup.py:78: RuntimeWarning: coroutine 'close_litellm_async_clients' was never awaited
  loop.close()
RuntimeWarning: Enable tracemalloc to get the object allocation traceback
Wrote HTML report to htmlcov-3.12/index.html

```

tests/unit/distribution/routers/test_routing_tables.py

@@ -8,6 +8,8 @@
 
 from unittest.mock import AsyncMock
 
+import pytest
+
 from llama_stack.apis.common.type_system import NumberType
 from llama_stack.apis.datasets.datasets import Dataset, DatasetPurpose, URIDataSource
 from llama_stack.apis.datatypes import Api
@@ -78,6 +80,9 @@ class SafetyImpl(Impl):
     async def register_shield(self, shield: Shield):
         return shield
 
+    async def unregister_shield(self, shield_id: str):
+        return shield_id
+
 
 class DatasetsImpl(Impl):
     def __init__(self):
@@ -191,12 +196,42 @@ async def test_shields_routing_table(cached_disk_dist_registry):
     await table.register_shield(shield_id="test-shield", provider_id="test_provider")
     await table.register_shield(shield_id="test-shield-2", provider_id="test_provider")
     shields = await table.list_shields()
-
     assert len(shields.data) == 2
+
     shield_ids = {s.identifier for s in shields.data}
     assert "test-shield" in shield_ids
     assert "test-shield-2" in shield_ids
 
+    # Test get specific shield
+    test_shield = await table.get_shield(identifier="test-shield")
+    assert test_shield is not None
+    assert test_shield.identifier == "test-shield"
+    assert test_shield.provider_id == "test_provider"
+    assert test_shield.provider_resource_id == "test-shield"
+    assert test_shield.params == {}
+
+    # Test get non-existent shield - should raise ValueError with specific message
+    with pytest.raises(ValueError, match="Shield 'non-existent' not found"):
+        await table.get_shield(identifier="non-existent")
+
+    # Test unregistering shields
+    await table.unregister_shield(identifier="test-shield")
+    shields = await table.list_shields()
+
+    assert len(shields.data) == 1
+    shield_ids = {s.identifier for s in shields.data}
+    assert "test-shield" not in shield_ids
+    assert "test-shield-2" in shield_ids
+
+    # Unregister the remaining shield
+    await table.unregister_shield(identifier="test-shield-2")
+    shields = await table.list_shields()
+    assert len(shields.data) == 0
+
+    # Test unregistering non-existent shield - should raise ValueError with specific message
+    with pytest.raises(ValueError, match="Shield 'non-existent' not found"):
+        await table.unregister_shield(identifier="non-existent")
+
 
 async def test_vectordbs_routing_table(cached_disk_dist_registry):
     table = VectorDBsRoutingTable({"test_provider": VectorDBImpl()}, cached_disk_dist_registry, {})