Finish fixture cleanup and expand server tests

[nipun172006]

Summary

• remove the remaining reproducible 26 MB binary fixture and its stale demo links
• remove the submission salutation and roll number from the source module docstring
• generate a deterministic multi-chunk streaming fixture at test time instead of storing a large binary
• add streaming and bounded-queue concurrency tests while preserving the default queue behavior

Verification

• python3 -m unittest discover -s tests -v — 12 tests pass
• repository contains no tracked file larger than 1 MB
• submission salutation and roll-number markers are absent

No history rewrite or force-push is involved.