Fix memory leak potential by not calling op.cancel()

[sudorandom]

Ensures the Transcoder request sub-context is always cancelled and correctly propagated to downstream handlers.

Changes

• Added defer op.cancel() to ServeHTTP to prevent context leaks.
• Updated ServeHTTP to pass op.request (carrying the sub-context) to downstream handlers on
  pass-through and unknown route paths.

[emcfarlane]

Thanks for the fix! Can you clarify: "Updated ServeHTTP to pass op.request (carrying the sub-context) to downstream handlers on pass-through and unknown route paths."?