refactor(bigtable): address igor nits on Session lifecycle

Five nits from the igor-reviewer-agent review of PR #20215:

- handleGoAway's per-GOAWAY drain goroutine now derives its 30s timeout
  from s.stream.Context() instead of context.Background(). Pool teardown
  cancels the stream ctx, which now unblocks the drain wait cleanly
  instead of hanging up to 30s on a detached background ctx.
- handleOpenSession: doc comment now spells out that the response body
  is intentionally not consumed (PeerInfo comes from the stream Header,
  not the reply payload). Documents the '_ *spb.OpenSessionResponse'
  signature so a future reader doesn't wonder if fields were missed.
- peerInfoExtracter → extractPeerInfo. Mutator, not a noun; and
  "extracter" is a non-standard spelling. Rename in impl + all test
  callers.
- notifyClosing docstring adds the OnActive-skip caveat: if a session
  tears down while still Starting (Close/ForceClose before OpenSession
  completes), OnActive never runs but OnClosing/OnClose still do.
  SESSION_SPEC #4 orders hooks that fire, not hooks that don't.
- readLoop gains a deferred panic-recover that records a debug tag and
  ForceCloses with REASON_ERROR + the panic value in the description.
  Avoids losing the readLoop goroutine silently if
  handleSessionResponse panics on a malformed frame. New tag constant
  tagSessionReadLoopPanic in debug_tracer.go.