fix: raise ImportError during PDFKnowledgeSource instantiation when pdfplumber is missing

- Add __init__ method to PDFKnowledgeSource that checks PDFPLUMBER_AVAILABLE
- Update test_optional_dependencies.py to expect ImportError during instantiation
- Fixes test_optional_pdf_import_error in test_lite_installation.py
- Ensures consistent behavior across all optional dependency implementations

Co-Authored-By: João <joao@crewai.com>

tests/test_optional_dependencies.py

@@ -44,10 +44,8 @@ class TestOptionalDependencies:
                 test_file.touch()
                 
                 try:
-                    pdf_source = PDFKnowledgeSource(file_paths=["test.pdf"])
-                    
                     with pytest.raises(ImportError) as exc_info:
-                        pdf_source._import_pdfplumber()
+                        PDFKnowledgeSource(file_paths=["test.pdf"])
                     
                     assert "pdfplumber is required" in str(exc_info.value)
                     assert "crewai[knowledge]" in str(exc_info.value)