fix(filesystem): ignore final newline in tail output

[Sean-Kenneth-Doherty]

Summary

• Fix tailFile so a normal final newline is not returned as an empty trailing line.
• Add regression coverage that asserts tailFile content instead of only checking handle cleanup.
• Keep intentional blank lines intact by removing only the split sentinel from the EOF chunk.

Root Cause

tailFile splits the final chunk on \n. For text files ending in a newline, split("\n") produces a final empty sentinel, which the reverse reader counted as the last line. That made read_text_file with tail: 1 return an empty string for ordinary POSIX-style files like line1\nline2\nline3\n.

Validation

• npm exec -w src/filesystem -- vitest run __tests__/lib.test.ts
• npm run build -w src/filesystem

[Sean-Kenneth-Doherty]

Marked ready after rechecking the patch locally.

Why this is ready:

• the fix is scoped to tailFile only
• it preserves intentionally blank lines and removes only the EOF split sentinel from the final chunk
• regression tests cover tail: 1 and tail: 2 on ordinary newline-terminated text

Validation:

• npm exec -w src/filesystem -- vitest run __tests__/lib.test.ts - 46 passed
• npm run build -w src/filesystem - passed
• hosted Python/TypeScript test and build checks are passing