feat(live-runner): forward orchestrators through reserve_session

Wire the orchestrators arg from reserve_session into runner_selector so
callers can pass a list (or comma-delimited string) of orchestrator URLs
for runner discovery.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: rickstaa

src/livepeer_gateway/selection.py

@@ -267,6 +267,7 @@ async def runner_selector(
 
 async def reserve_session(
     *,
+    orchestrators: Optional[Sequence[str] | str] = None,
     signer_url: Optional[str] = None,
     signer_headers: Optional[dict[str, str]] = None,
     discovery_url: Optional[str] = None,
@@ -277,6 +278,7 @@ async def reserve_session(
     payment_interval: float = 3.0,
 ) -> LiveRunnerSession:
     cursor = await runner_selector(
+        orchestrators=orchestrators,
         signer_url=signer_url,
         signer_headers=signer_headers,
         discovery_url=discovery_url,