Add Agent Builder APIs to Kibana diagnostics

The following APIs can be helpful to add to the Kibana diagnostics for Agent Builder.

### Core APIs

**Listing agents:** GET kbn://api/agent_builder/agents

**Listing tools:** GET kbn://api/agent_builder/tools

**Listing conversation:** GET kbn://api/agent_builder/conversations

### Less important for the diagnostic bundle (just a FYI that these APIs exist):

**Getting histories from individual conversations:** GET kbn://api/agent_builder/conversations/eddb643a-58e7-43e3-a7d8-cf1eda4040f5 (This is more of a FYI. List conversations above only return the ID/metadata of the conversations. It will not return the reasoning steps, ES|QL results, LLM response, etc. There could be a large number of conversation histories so I would not suggest getting histories from every conversation by default. We should ask the customer to help identify the problem conversation and then ask them to run this API manually.

**List attachments from individual conversations:** GET kbn://api/agent_builder/conversations/eddb643a-58e7-43e3-a7d8-cf1eda4040f5/attachments?include_deleted=true (This is more of a FYI. List conversations above only return the ID/metadata of the conversations. It will not return the reasoning steps, ES|QL results, LLM response, etc. There could be a large number of conversation histories so I would not suggest getting attachments from every conversation by default. We should ask the customer to help identify the problem conversation and then ask them to run this API manually.

**Get A2A agent card:** GET kbn://api/agent_builder/a2a/{agentId}.json (We could run this against each agent returned from listing agents).

NOTE: All the API calls above are space-aware, requests like `GET kbn://api/agent_builder/agents` when running within the Kibana Dev Console, direct HTTP requests (e.g., curl) will require specifying the space name in the endpoint in order to return the agents/tools/etc. within a specific space. For example:

```
curl -X GET -H "Authorization: ApiKey $ELASTIC_API_KEY" "/s//api/agent_builder/agents"
```

Otherwise, it would only return the Agent Builder artifacts from the default space.