fix(framework): Bound task log uploader shutdown

[tanertopal]

What changed

• bound task log-upload RPCs with the task-worker deadline
• retain definitively unavailable batches while stopping
• discard deadline-expired batches whose delivery is ambiguous
• keep the stop sentinel from dropping a retained final batch

Why

Log upload must not consume the full process shutdown window. Because log appends are non-idempotent, a timed-out batch cannot safely be retried when the server may already have accepted it.

Stack

Umbrella/reference: #7895 — Prevent task executor shutdown deadlock

• Previous: #7917 — Bound task heartbeat shutdown
• This PR: fix(framework): Bound task log uploader shutdown #7918 — Bound task log uploader shutdown
• Next: #7919 — Stop heartbeat executors before completion

Merge the stack in order. After the previous PR merges, retarget this PR to main.

Validation

• pytest py/flwr/common/logger_test.py
• Ruff and Black on the changed logger modules
• git diff --check