fix(server): proxy namespace policy in tool context

[Mijamind719]

Summary

• add a ToolContext.namespace_policy proxy to its wrapped RequestContext
• cover the proxy behavior in identity tests

Related Issue

• Refs Bug: ToolContext missing namespace_policy property breaks search tool #1667

Tests

• /Users/quemingjian/Source/OpenViking/.venv/bin/python -m ruff check openviking/server/identity.py tests/server/test_identity.py
• /Users/quemingjian/Source/OpenViking/.venv/bin/python -m pytest -q tests/server/test_identity.py

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