[FEAT] add rampup for GuideLLM concurrent benchmarks

[VincentG1234]

Summary

Add optional benchmark.rampup to study configs, forwarding GuideLLM's --rampup flag so concurrent benchmarks can ramp load linearly to target concurrency instead of starting at full rate.

Why

Sudden full-concurrency load can skew benchmark metrics (cold caches, queue buildup, OOM risk). GuideLLM supports a ramp-up period; this fork already exposes warmup and cooldown but not rampup, so users could not control how load increases at the start of a run.

What changed

• auto_tune_vllm/benchmarks/config.py — add optional rampup field; validate > 0 in __post_init__
• auto_tune_vllm/benchmarks/providers.py — pass --rampup to GuideLLM CLI when set
• tests/benchmarks/test_guidellm_command.py — CLI construction and validation tests for rampup
• docs/configuration.md — document rampup (seconds, included in metrics, unlike warmup)
• examples/study_config.yaml, examples/study_config_minimal.yaml — commented example
• README.md — fork changelog entry

How tested

• ruff check .
• pytest -v tests/benchmarks/test_guidellm_command.py (11 passed)
• Manual E2E (maintainer): auto-tune-vllm optimize with benchmark.rampup: 10 and verify GuideLLM receives --rampup 10

Risks / limitations

• Requires a GuideLLM version that supports --rampup (same pattern as existing warmup/cooldown flags).
• Ramp-up requests are included in reported metrics; only warmup/cooldown exclude phases from measurement.
• No interaction with fractional warmup/cooldown sum validation (rampup is always absolute seconds).

Links

• Follows #24 (warmup/cooldown) and #27 (sample_requests) benchmark config extensions.