Move pypdf, python-docx, openpyxl, and python-pptx imports from module
level into the _extract_pdf / _extract_docx / _extract_xlsx /
_extract_pptx functions that actually use them. These four libraries
became core dependencies in v0.1.5.post2 (~25 MB combined) and were
paying the import cost on every nanobot startup even when no document
parsing was needed for the session.
The module-level SUPPORTED_EXTENSIONS set and the extract_text()
dispatch stay as-is; the "[error: <lib> not installed]" branches move
from the old module-level None sentinels into the corresponding
extractor's try/except ImportError block. Behavior for the error
message and for successful parses is identical.
All 20 tests in tests/test_document_parsing.py pass unchanged.
Fixes#3422
