[bug]: update() strips None values via exclude_none=True, making it impossible to clear nullable fields

Summary

The update() method on every resource class strips None values from the payload via model_dump(exclude_none=True),
making it impossible to clear any nullable field on an existing work item (or any other resource using the same
pattern).

Reproducer

# Setup: work item with target_date = "2026-05-17"
data = UpdateWorkItem(target_date=None)
client.work_items.update(
    workspace_slug=ws, project_id=p, work_item_id=w, data=data, 
) 
# Expected: target_date cleared to null
# Actual: silent no-op — target_date still 2026-05-17
# Wire body sent: {} (None stripped by exclude_none=True)

Root cause

plane/api/work_items/base.py line 102 (same pattern in other resources):

response = self._patch(
    f"{workspace_slug}/projects/{project_id}/work-items/{work_item_id}",
    data.model_dump(exclude_none=True),
)

exclude_none=True conflates two distinct user intents: "do not update this field" and "clear this field to null."

Impact

Affects every nullable field on every resource using this pattern: target_date, start_date, parent, point,
description_html, assignees (back to empty), etc. Currently the only workaround is clearing in the Plane web UI.

Proposed fix (two viable shapes)

A. Switch to exclude_unset=True + explicit None for clears

• Caller passes None to mean "clear," and omits the field entirely to mean "don't change."
• Requires the data model to distinguish unset from None (Pydantic v2 supports this natively).
• Breaking change for callers that currently pass None expecting "don't change."

B. Add explicit clear: list[str] parameter to update() methods

• Caller passes e.g. clear=["target_date", "parent"] and the wrapper injects null for those fields after
  model_dump.
• Non-breaking; opt-in.
• More verbose but unambiguous.

Happy to send a PR with whichever shape the maintainers prefer.

Environment

• plane-sdk 0.2.10 (also present on main HEAD as of 2026-05-17)
• Plane Cloud workspace
• Discovered via plane-mcp-server's update_work_item tool, but the bug is in the SDK, not the MCP wrapper.

从 plane/api/work_items/base.py 第 102 行附近开始，然后在其他资源类中搜索相同的 model_dump(exclude_none=True) 模式。在选择两个提议方案中的一个之前，比较更新模型中未设置字段和显式 None 的表示方式。完成标准是：在受影响的资源中，可以清除 nullable 字段，同时不会将省略的字段变成更新。