Evaluate before_provider_headers for telemetry and gateway attribution

[spences10]

Parent: #287

Upstream PR: earendil-works/pi#6350

Upstream adds a before_provider_headers extension hook for mutating outgoing provider request headers.

Potential my-pi impact:

• @spences10/pi-telemetry
• @spences10/pi-observability
• future gateway/billing/session attribution

Acceptance criteria:

• Decide whether telemetry/observability should attach trace/session headers through this hook.
• Avoid duplicating provider request plumbing if the SDK hook is enough.
• Document any opt-in behavior and privacy implications.