Filter FILE placeholder hints

[InkiChang]

What

• Ignore instructional FILE placeholders such as [FILE:filepath] when extracting attachments.
• Deduplicate extracted FILE paths in the shared chat frontend helper.
• Reuse the shared extractor in the Feishu frontend.

Why

• The FILE hint shown to agents can be echoed back literally and was being treated as a missing attachment path in Feishu.
• Filtering placeholder examples prevents frontends that use the shared extractor from trying to send non-existent files.

Testing

• python3 -m unittest tests.test_chatapp_common
• python3 -m py_compile frontends/chatapp_common.py frontends/fsapp.py tests/test_chatapp_common.py