add unit tests for build scripts

[tejas0077]

Adds unit tests for 4 build scripts as requested by @romanlutz in #1569.

Closes #1570

• test_prepare_package.py — 9 tests covering npm lookup, install/build failures, and frontend copy logic
• test_validate_docs.py — 14 tests covering TOC parsing, missing file detection, and orphaned file checks
• test_generate_rss.py — 10 tests covering date extraction and blog markdown parsing
• test_check_links.py — 13 tests covering URL extraction, fragment stripping, and relative URL resolution

Added 46 new unit tests across 4 test files covering all 4 build scripts.
All 65 tests in tests/unit/build_scripts/ pass locally:

python -m pytest tests/unit/build_scripts/ -v

65 passed

No documentation changes required — this is a pure test addition.
No JupyText changes required — no notebook modifications.

[tejas0077]

Closing in favour of cleaner PR #1642