Manager: stream handler defaultdicts grow unbounded across kernel lifecycle

## Objective

The four per-kernel stream defaultdicts (stream_pty_handlers, stream_execute_handlers, stream_proxy_handlers, stream_stdin_socks in src/ai/backend/manager/api/rest/stream/handler.py) never delete their outer keys. handle_kernel_terminating() uses a bare stream_stdin_socks[stream_key] access, so defaultdict.__missing__ inserts an empty key on every kernel termination, including kernels that never opened a stream.

## Evidence (live worker, Linux + memray)

- Real session create->terminate cycles left rest/stream/handler.py and defaultdict __missing__ frames in the memray --leaks (un-freed) set.
- Shutdown cleanup only iterates .values(); outer dict keys are never removed.

## Fix

- Use .get() instead of bare defaultdict access in handle_kernel_terminating.
- Delete all four per-kernel dict keys on kernel termination.

## Acceptance Criteria

- Per-kernel dict keys are removed on termination.
- Stream defaultdict sizes are bounded by the number of live kernels.

JIRA Issue: BA-6804

Start in src/ai/backend/manager/api/rest/stream/handler.py, inspecting the four per-kernel stream defaultdicts and handle_kernel_terminating(). Verify cleanup for kernels with and without opened streams; done means all four outer keys are removed on termination and their sizes remain bounded by live kernels.

Written by the indexing model from the issue text.