feat(parser): skip malformed JSONL lines; add tests

[annenpolka]

What

• Skip malformed JSONL lines during parsing
• Add test to fix desired behavior (TDD: Red→Green)
• Keep existing behavior for empty files and directory parsing

Why

• Real-world logs may include partially written or corrupt lines. Parsing should be resilient and continue for valid lines.

How (Implementation)

• On json.Unmarshal failure for a line, continue instead of returning error.
• Test: file with valid1\ninvalid\nvalid2 should produce 2 messages without error.

Tests

• make fmt vet test all pass locally.

Notes

• Followed repository guidelines (table-driven tests style, co-located tests).