Previously the tokio runtime was stashed in a hidden attribute on the
reactor object, installed lazily by whichever Rust code first needed it,
and started via `callWhenRunning`.
Instead, we create a `RustRuntime` (accessible via
`HomeServer.get_rust_runtime()`) that holds any per-reactor Rust state,
such as the tokio runtime. It is constructed lazily on use. Rust
consumers (`HttpClient`, `VersionsHandler`, the Python DB pool wrapper)
now receive the runtime or reactor handle explicitly, and the
`reactor.run()` / manual-startup workarounds in tests are no longer
needed.
We also add helper wrappers in Rust for `Reactor` and `HomeServer` that
exposes the needed functionality.
The aim is to allow us to have a Rust-side clock (mainly to get the
current time), that respects the unit test per-reactor time management.
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Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
