fix(sa-bench): shard high-concurrency loadgen across frontends

[YAMY1234]

Summary

• Add repeated --api-url support to SA-Bench and round-robin requests across multiple frontend endpoints while sharing one aiohttp session.
• Teach bench.sh to populate direct frontend targets from SA_BENCH_API_URLS or /logs/nginx.conf when SA_BENCH_SHARD_FRONTENDS=true.
• Pass benchmark.env through benchmark runners so recipes can enable sharded SA-Bench without using custom commands.

Why

High-concurrency disaggregated serving runs can exhaust ephemeral ports when a single load generator connects to one host:port tuple. Sharding request traffic across the deployed frontend endpoints avoids that client-side collapse without changing backend topology.

For conc=32768
Before

After

Validation

• uv run ruff check src/srtctl/benchmarks/base.py src/srtctl/benchmarks/custom.py src/srtctl/benchmarks/scripts/sa-bench/backend_request_func.py src/srtctl/benchmarks/scripts/sa-bench/benchmark_serving.py
• uv run python -m py_compile src/srtctl/benchmarks/scripts/sa-bench/backend_request_func.py src/srtctl/benchmarks/scripts/sa-bench/benchmark_serving.py
• uv run pytest tests/test_benchmarks.py tests/test_frontends.py
• CoreWeave validation: job 3487, 6P1D DEP24, conc=36864, sharded SA-Bench, completed 110592/110592 with error=0 and api_urls=9.
• CoreWeave validation: job 3528, 4P1D DEP24, conc=32768, sharded SA-Bench, completed 98304/98304 with error=0 and api_urls=9.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 87.50000% with 2 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@404ad25). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/srtctl/cli/mixins/benchmark_stage.py	81.81%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
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  Coverage        ?   65.10%           
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  Files           ?       67           
  Lines           ?     8228           
  Branches        ?        0           
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  Hits            ?     5357           
  Misses          ?     2871           
  Partials        ?        0           

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