Fix ramp halt timing and add ramp window coverage

[hemanth-asirvatham]

Motivation

• Ensure the initial concurrency ramp only gets frozen if a rate-limit or connection error occurs during the active ramp window, and not after the ramp completes.
• Make parallelization status updates more informative by surfacing per-interval deltas for rate-limit and connection errors.
• Provide deterministic behavior and coverage so the ramp behavior is well-tested for both in-window and post-window error cases.

Description

• Prevent _halt_ramp_up() from taking effect after the ramp window by returning early when time.time() >= ramp_up_end_time.
• Immediately call _halt_ramp_up() from worker error handlers on first observed rate-limit or connection errors to lock the ramp while it is still active.
• Add _maybe_emit_ramp_complete() (and call it from the embeddings worker loop) to log/print a ramp-complete message when the ramp window elapses.
• Include per-period denominators and delta counts for rate_limit_errors and connection_errors in emit_parallelization_status() and track/reset rate_limit_errors_since_last_status in the reporter loop.
• Improve first-error logging for rate-limit and connection events to also print to stdout when message_verbose is enabled.
• Tests updated: make test_ramp_up_halts_on_rate_limit assert that a halting message is printed and add test_ramp_up_does_not_halt_after_window to verify that a late error (after the ramp window) does not halt the ramp.

Testing

• Ran the focused tests with python -m pytest tests/test_parallelization_tuning.py, and all tests passed (8 passed in 4.52s).
• Changes exercised include the new halting logic and the added post-window coverage test which both succeeded.

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Codex Task

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