[1/6] Add Python goal routing foundation

[aibrahim-oai]

Why

Goal continuation turns are emitted by the existing runtime as separate physical turns. The Python SDK needs private thread-scoped routing before it can present those notifications as one logical operation, without changing ordinary turn routing or the app-server protocol.

What

• add private goal operation state and thread-scoped notification routing
• add internal wrappers for the existing thread/goal/clear and thread/goal/set RPCs
• include existing goal notifications in the SDK notification union
• preserve ordinary turn-ID routing unchanged
• add focused routing coverage

This PR does not expose a public goal API. It is the first of six focused PRs in the Python goal operations stack.

Test plan

• online CI, including the Python SDK suite
• focused typed-notification routing coverage

[aibrahim-oai]

This change is part of the following stack:

• [1/6] Add Python goal routing foundation #27110 ◀
  • [2/6] Add private Python goal operations #27111
    • [3/6] Add private Python goal-aware turn handles #27112
      • [6/6] Add Python goal cleanup integration coverage #27113

_{Change managed by git-spice.}