feat: extract markdown for PDF pages with password

[aynugek]

Summary by cubic

Adds password support to per-page Markdown extraction so encrypted PDFs can be processed. Previously extract_pages_markdown failed on encrypted PDFs; now callers can pass a password, and an empty password is tried by default. Wrong or missing passwords return an Encrypted error.

API changes

• Rust: added extract_pages_markdown_with_password and extract_pages_markdown_mem_with_password; existing functions delegate with None.
• Python: extract_pages_markdown and extract_pages_markdown_bytes accept an optional password; stubs and docs updated.
• Node (N-API): extract_pages_markdown and extract_pages_markdown_async accept an optional password; existing two-arg calls continue to work.

Notes

• Encrypted files without a correct password return PdfError::Encrypted, propagated to Python and Node.
• Rust and Python API docs updated; new integration test covers decryption and error cases.

^{Written for commit 4c90d4d. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 7 files

<sub>Shadow auto-approve: would auto-approve. Adds optional password support for extract_pages_markdown in Rust core and Python/Node bindings with a new integration test. Backward compatible, documented, and tested; no risky changes are visible, so auto-approval.

Re-trigger cubic</sub>