th-137b91: Python server durable-executor selection seam (Q parity)

[brentrager]

Problem

Polyglot-parity item Q (ADR-030 durable execution) for the Python server.
TurnRunner called SmoothAgent.run_stream directly, so there was no single place
a durable backend could be selected — unlike the Rust server, whose turn_executor
(runner.rs) picks the executor a turn runs on.

Solution

Route each turn through the engine's AgentExecutor seam, selected once in
select_turn_executor — the Python analog of the Rust turn_executor.

• Dependency injection: a durable backend is passed to TurnRunner(executor=...)
  as an opaque AgentExecutor, so the server keeps no hard dependency on the
  Temporal package. (The durable backend itself is the Python
  smooai-smooth-operator-temporal package, item I.)
• Env-gated: SMOOTH_AGENT_DURABLE_EXECUTOR (1/true/on/yes) opts in. The
  injected backend is used only when the env asks; otherwise — and whenever nothing
  is injected — the turn runs on InProcessExecutor, a verbatim delegation to
  run_stream (behavior unchanged). Asking for durable mode with nothing injected
  warns and falls back rather than silently pretending a turn is durable.
• durable_requested(value) is split out so the parse is testable without mutating
  process-global state.

Deliberate difference from Rust: there an injected executor wins unconditionally;
here the env var is the explicit opt-in, so an injected backend can never silently
take over a deployment that didn't ask. Noted inline.

Verification

• New test_durable_executor_seam.py: durable_requested parse table;
  select_turn_executor (injected-when-on, ignored-when-off, warn-fallback,
  fresh-per-call); and two real turns driven through a fake injected executor
  (env on → durable used; off → in-process). 19 passed.
• Full server suite: 350 passed — the run_stream → execute_streaming swap
  regresses nothing.
• ruff check + ruff format --check clean.

🤖 Generated with Claude Code

https://claude.ai/code/session_01YbN45JeWDbcjvFqGJvmVD3

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🦋 Changeset detected

Latest commit: 8873083

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@smooai/smooth-operator	Patch
@smooai/smooth-operator-web-chat-example	Patch

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