feat(code): host Python extensions in the agent runtime (#5633)

Related #5556

Hosts extension tools, middleware, and virtual filesystem backends in
the agent runtime.

---

Layer 3 of 4. Static registrations are applied during graph
construction, late tools remain callable, and late graph-bound
registrations signal that a restart is required. Backend routes are
validated against internal and sandbox boundaries, shell execution
remains on the default backend, and the persistent server lifespan owns
teardown and a loopback-only provenance endpoint. User-facing trust and
inspection UX are deliberately excluded.

## Stack

1. [#5631 — API, registry, and
discovery](https://github.com/langchain-ai/deepagents/pull/5631)
2. [#5632 — configuration, trust, and
loading](https://github.com/langchain-ai/deepagents/pull/5632)
3. [#5633 — agent hosting and
lifecycle](https://github.com/langchain-ai/deepagents/pull/5633)
4. [#5634 — trust and inspection
UX](https://github.com/langchain-ai/deepagents/pull/5634)