TaskType enum missing AGENT, GET_AGENT_CARD, CANCEL_AGENT, PULL_WORKFLOW_MESSAGES

## Summary

Four task types added to the Conductor server in 3.32.0-rc.9 (PR conductor-oss/conductor#1288)
are absent from the SDK's `TaskType` enum. Calling `TaskType.of("AGENT")` silently returns
`USER_DEFINED` instead of the correct value.

## Server baseline

Conductor **3.32.0-rc.9**

## Missing values

The server's `TaskType.java` has:
```java
PULL_WORKFLOW_MESSAGES,
AGENT,
GET_AGENT_CARD,
CANCEL_AGENT;
```

The SDK's `TaskType.java` stops at `CALL_MCP_TOOL` — all four values are absent.

## Impact

- `TaskType.of("AGENT")` returns `USER_DEFINED` — silently wrong
- No builder classes can be typed to these task types
- SDK users writing AGENT workflows must hard-code raw strings

## Evidence

**Static:** Compared server `TaskType.java` against SDK `TaskType.java`.

**Live (2026-07-16):** A raw `WorkflowTask` with `"type": "AGENT"` was registered
against Conductor OSS 3.32.0-rc.9. Server accepted the definition, confirming the type is
valid server-side — the gap is SDK-only.

## Proposed fix

Add to the SDK `TaskType` enum:
```java
PULL_WORKFLOW_MESSAGES,
AGENT,
GET_AGENT_CARD,
CANCEL_AGENT;
```

Builder classes for AGENT, GET_AGENT_CARD, and CANCEL_AGENT should follow once the
agent task API is stable.

## Related

Discovered during systematic SDK audit against Conductor OSS 3.32.0-rc.9.
Same gap exists in the Python SDK (conductor-oss/python-sdk#431).
Server PR: conductor-oss/conductor#1288

Compare the SDK's TaskType.java with the server's TaskType.java at Conductor 3.32.0-rc.9, starting with the SDK enum entry that currently ends at CALL_MCP_TOOL. Verify that TaskType.of("AGENT") and the other three names resolve to their corresponding enum values, with no builder work required until the agent task API is stable.

Written by the indexing model from the issue text.