Fix yaml syntax for threshold with warning_range

[xlamorlette-datadog]

As stated in IDMPL-457 Nginx: master CI: check-slo-breaches broken, since April 16, the check-slo-breaches CI job used to fail.
This was due to an invalid yaml syntax when using a pre-threshold warning_range.

Example of failure before the fix:

$ bp-runner $SLO_FILE
Running on a Linux distro
bp-runner v2026.5.5
mapping values are not allowed here
  in ".gitlab/bp-runner.fail-on-breach.yml", line 17, column 30
2026-05-19 08:26:43,139 [ERROR] bp-runner.src.execution.runner - 'NoneType' object has no attribute 'setup'

Success in the benchmark pipeline manually triggered in this PR:

$ bp-runner $SLO_FILE
Running on a Linux distro
bp-runner v2026.5.5
2026-05-19 12:16:52,851 [INFO] bp-runner.src.templates.fail_on_breach - Generating Markdown threshold comparison report for CI logs...
Benchmark execution time: 2026-05-19 12:16:54
Comparing candidate commit 50bd3ca156cea2e081e58acc9207df9e06e154cf in branch `xlamorlette/fix-check-slo-breaches` with performance thresholds.
#### high_load--only-tracing
- 🟩 `throughput` seen in benchmark [3979.843op/s; 4084.471op/s]; SLO is > 3700.000op/s
#### normal_operation--only-tracing
- 🟨 `agg_http_req_duration_p50` seen in benchmark [114.488µs; 114.730µs]; SLO is < 116.000µs (warning threshold is 113.680µs)

[datadog-prod-us1-5]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 67.62% (+0.00%)

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