fix(helm): apply trafficDistribution setting to SingleBinary service

[railgun-0402]

What this PR does / why we need it:
The singleBinary.service.trafficDistribution value was documented in
values.yaml but never applied to the SingleBinary Service template,
causing the setting to be silently ignored.

This PR adds the same trafficDistribution rendering that already exists
in the Write, Read, Backend, and other service templates.
Both singleBinary.service.trafficDistribution and the global fallback
loki.service.trafficDistribution are now respected.

Which issue(s) this PR fixes:
Fixes #20428

Special notes for your reviewer:
One-line addition following the exact same pattern used in
templates/write/service-write.yaml (and read, backend, etc.).

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here

[CLAassistant]

All committers have signed the CLA.

[TheRealNoob]

LGTM

cc @JStickler