fix: leave proposals open when the RFP check cannot read them

Summary

A failed proposal download now fails the RFP category check and leaves the PR open. Previously, the check continued after a download error and could close a valid proposal as though it contained no RFP category.

Validation

node --test tests/rfp-category-check.test.cjs passes three tests against the workflow's actual JavaScript: failed reads cause no GitHub mutations, a valid RFP is labelled, and a successfully read proposal without a category still follows the existing closure rule. The failed-read test fails before the fix.

This is workflow maintenance; the funding-proposal application fields are not applicable.

Post-Deploy Monitoring & Validation

If a proposal read fails after merge, the check should fail without closing or labelling the PR. Maintainers can rerun it after the read failure clears. No live proposal was modified during testing.