Add stack tests to benchmarks

[podocarp]

Added the stack tests to the e2e benchmarks

Config: 10s duration, 3s warmup per endpoint

Negative diff = slower with SDK enabled (RECORD mode) vs disabled.

Django + PostgreSQL

Benchmark	Baseline	With SDK	Diff
GET /db/register-jsonb	994.25/s	312.64/s	-68.6%
GET /health	3151.13/s	709.91/s	-77.5%

Django + Redis

Benchmark	Baseline	With SDK	Diff
DELETE /cache/delete/counter	2788.42/s	440.44/s	-84.2%
DELETE /cache/delete/test_key	2800.57/s	441.63/s	-84.2%
GET /cache/get/counter	3002.91/s	521.11/s	-82.6%
GET /cache/get/test_key	2974.06/s	516.98/s	-82.6%
GET /health	3029.68/s	734.72/s	-75.7%
GET /redis/direct	2726.18/s	350.28/s	-87.2%
GET /redis/pipeline	2684.29/s	361.25/s	-86.5%
GET /session/get	2964.57/s	737.40/s	-75.1%
POST /cache/incr/counter	2745.65/s	397.97/s	-85.5%
POST /cache/set	2861.07/s	502.72/s	-82.4%
POST /session/clear	2945.69/s	738.52/s	-74.9%
POST /session/set	1765.88/s	222.46/s	-87.4%

FastAPI + PostgreSQL (psycopg3)

Benchmark	Baseline	With SDK	Diff
GET /db/async-context	1312.35/s	203.61/s	-84.5%
GET /db/pipeline	1124.66/s	238.65/s	-78.8%
GET /db/query	1998.58/s	321.29/s	-83.9%
GET /db/sync-fallback	830.33/s	291.85/s	-64.9%
GET /db/transaction	2151.74/s	N/A	-
GET /health	3605.16/s	727.92/s	-79.8%
POST /db/insert	2056.55/s	N/A	-

[cubic-dev-ai]

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