Add AI gateway workload benchmark with 4-phase concurrency ramp and pricing

[dtice25]

Summary

Adds a new AI gateway workload benchmark that ramps concurrency through four fixed levels (1, 8, 16, 32) and reports sustained throughput, p50/p95/p99 latency, TTFT, error/timeout rates, max-sustained concurrency, and a per-provider pricing comparison.

• benchmarks/ai-gateway/ai-gateway-workload.bench.ts: Anthropic-family entry point with groupBy: 'round' and groupBy: 'concurrency' scoring.
• benchmarks/ai-gateway/workload-task.ts: sustained p-limit request pool over sendAndMeasure, returning per-level aggregates plus raw totalMs/ttftMs arrays for scoring.
• benchmarks/ai-gateway/workload-results.ts: legacy JSON writer that detects maxSustainedConcurrency (error rate < 5%, p95 total < 30s, throughput not degraded > 25% from previous level) and attaches cost metrics plus a valueScore.
• benchmarks/ai-gateway/pricing.ts: token-pricing map seeded with Claude Haiku 4.5 list price ($1/$5 per 1M input/output) and per-provider confidence flags.
• benchmarks/ai-gateway/phase-probe.ts: exports buildRequestBody, sendAndMeasure, RawProbeOutcome, and adds extractInputTokens/inputTokens so the workload can compute cost from real token counts.
• package.json: adds bench:ai-gateway-workload and a few --provider example scripts.

Run with:

pnpm bench:ai-gateway-workload
pnpm bench:ai-gateway-workload:openrouter

Verified with pnpm typecheck, pnpm -r --filter "./packages/**" build, and @benchsdk/runner / @benchsdk/client tests. An actual end-to-end run requires the AI gateway API keys and is not included in this change.

Link to Devin session: https://app.devin.ai/sessions/4ea7e5707b4e40e585c0e8befd7249b9
Requested by: @dtice25

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