fix: report the real cause when cel-python or tree-sitter fails to import

[Marc-cn]

Summary

Two import guards report "not installed" for any ImportError. While working on #573 that pointed at the wrong cause twice:

What was wrong	What darnit said
tree-sitter-language-pack 1.6.3 was installed, but its module was missing	"tree-sitter-language-pack is not installed"
cel-python was installed; celpy.c7nlib could not import packaging	"cel-python not installed. Install with: pip install cel-python"

In both cases the package was installed and the advice was wrong. This keeps the original error text:

• darnit_baseline/threat_model/parsing.py: the fallback get_language / get_parser raise "tree-sitter-language-pack could not be imported: ".
• darnit/sieve/cel_evaluator.py: the three guards raise "cel-python could not be imported: ".

Same failures, new messages:

cel-python could not be imported: No module named 'packaging.version'
tree-sitter-language-pack could not be imported: No module named 'tree_sitter_language_pack'

Follow-up to the note in #573 ("Including the original exception text in the message would help. Not changed here.").

Type of Change

• Bug fix (non-breaking change fixing an issue)
• New feature (non-breaking change adding functionality)
• Breaking change (fix or feature causing existing functionality to change)
• Documentation update
• Refactoring (no functional changes)

Framework Changes Checklist

• Updated framework spec (docs/architecture/framework-design.md) if behavior changed (message text only)
• Ran uv run python scripts/validate_sync.py --verbose and it passes

Testing

• Tests pass locally (uv run pytest tests/ -v): 5139 passed, 26 skipped (main: 5138)

• Added tests for new functionality (if applicable)

• Linting passes (uv run ruff check .)

• New tests/darnit/sieve/test_cel_import_failure.py: makes import celpy fail with a specific error and asserts the raised CELCompilationError carries it and no longer says "not installed". It fails without the fix.

• The parsing guard has no automated test. Its branch is marked # pragma: no cover, and exercising it means failing a module import and reloading the module inside the test session. I checked it by hand: with tree_sitter_language_pack blocked in sys.modules, the message is "tree-sitter-language-pack could not be imported: import of tree_sitter_language_pack halted; None in sys.modules".

No existing test asserts on the old wording.

AI assistance

• No AI assistance was used
• AI assistance was used

Claude Fable 5.1 wrote the changes and drafted this description. I ran every command and checked the results myself. The commit carries an `Assisted-by: Claude:claude-fable-5-1 trailer.

Additional Notes

I only changed the two guards I hit and read. There may be others with the same pattern elsewhere; I did not do a full sweep.