Add regression coverage for agent-context symlink updates

[Jiandong7]

Summary

• add regression coverage for update-agent-context.sh when the configured context file is a symlink
• assert the symlink is preserved and the managed section is written through to the target file

Validation

• python -m pytest tests\extensions\test_extension_agent_context.py -q (34 passed, 1 skipped locally because working bash is unavailable)
• git diff --check

Addresses #1464

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