No SDK support for AGENT, GET_AGENT_CARD, CANCEL_AGENT task types (added in server 3.32.0)

## Summary

Tested against: **Conductor OSS 3.32.0-rc.9**

Conductor server **3.32.0-rc.9** added three new task types for A2A conductor
agent workflows (conductor-oss/conductor PR #1288):

- `AGENT` — run an A2A conductor agent
- `GET_AGENT_CARD` — retrieve an agent card
- `CANCEL_AGENT` — cancel a running agent

None of these are present in the Python SDK's `TaskType` enum and no builder
classes exist for them.

## Current state

```python
from conductor.client.workflow.task.task_type import TaskType
hasattr(TaskType, "AGENT") # False
hasattr(TaskType, "GET_AGENT_CARD") # False
hasattr(TaskType, "CANCEL_AGENT") # False
```

## Requested

1. Add the three values to `TaskType` enum
2. Add builder classes (`AgentTask`, `GetAgentCardTask`, `CancelAgentTask`) once
the server-side inputParameters schema for each is stable

## Verified against

Conductor server **3.32.0-rc.9** (conductor-oss/conductor PR #1288).

Start in conductor.client.workflow.task.task_type.TaskType and compare its values with the three task types introduced by server 3.32.0-rc.9. Confirm the server-side inputParameters schemas are stable before assessing AgentTask, GetAgentCardTask, and CancelAgentTask. Done means the enum supports AGENT, GET_AGENT_CARD, and CANCEL_AGENT, with builder classes added when their schemas are stable.

索引モデルが issue の本文から書いたものです。