fix(sdk): allow clearing Python skill client defaults

[RerankerGuo]

Description | 描述

SkillClient.with_defaults() and AsyncSkillClient.with_defaults() currently
use None as the default value for every keyword argument. As a result, the
clone operation cannot distinguish an omitted argument from an explicit
request to clear an optional default:

client.with_defaults(task_id=None)

retains the existing task_id instead of removing it from subsequent request
bodies. This can keep stale optional isolation fields attached when an
application reuses a client for a broader scope.

This change:

• introduces an internal sentinel for omitted with_defaults() arguments;
• preserves existing defaults only when the corresponding argument is omitted;
• treats explicit None as a request to clear that default;
• applies the same behavior to synchronous and asynchronous skill clients;
• documents the distinction on both public clone methods.

The approach matches the sentinel pattern already used by the Python v3 data
client for clearable isolation fields.

Related Issue | 关联 Issue

L3/L4 Python SDK behavior audit finding; no existing issue or active PR modifies
the affected v3 Skill client files.

Change Type | 修改类型

• Bug fix | Bug 修复
• New feature | 新功能
• Documentation update | 文档更新
• Code optimization | 代码优化

Self-test Checklist | 自测清单

• Verified locally | 本地验证通过
• No existing features affected | 无影响现有功能

Verification | 验证方式

• PYTHONPATH=. /usr/bin/python3 -m pytest -q tests/test_v3_skill_defaults.py
  (2/2)
• PYTHONPATH=. /usr/bin/python3 -m pytest -q (2/2 collected tests)
• /usr/bin/python3 -m compileall -q tencentdb_agent_memory tests
• /usr/bin/python3 -m pip check
• /usr/bin/python3 -m pip wheel --no-deps . (wheel built and inspected)
• git diff --check

The regression tests exercise both sync and async clients. They verify that an
argument omitted from with_defaults() is inherited, while an explicitly
cleared value is absent from the next serialized /v3/skill/list request.

Additional Notes | 其他说明

This PR changes only clone-time handling of optional Skill client defaults.
Constructor behavior, per-request override behavior, transport sharing,
dependencies, and package metadata are unchanged. Existing calls that omit
arguments retain their current behavior; explicit None now performs the
clear operation that the public optional-value API could not previously
express.

[Maxwell-Code07]

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