fix(nntp): reject malformed overview ranges

[rissrice2105-agent]

Summary

• makes the NNTP overview range parser report malformed input instead of falling back to a broad 0..MaxInt64 range
• returns 501 syntax error for bad OVER / XOVER ranges
• adds coverage for open-ended valid ranges and malformed specs like abc, 1-abc, -5, 10-5, and 1-2-3

Fixes #45.

Validation

• go test ./internal/news/nntpd using Go 1.26.1