fix(middleware): avoid event-loop blocking in WorkspaceContextMiddleware onSystemPrompt (#2632)

# Background

`WorkspaceContextMiddleware.onSystemPrompt()` assembled workspace
context synchronously on the caller thread #2578. In WebFlux
deployments, that caller can be a Netty event-loop thread.
This PR fixes the issue by moving that workspace context assembly work
off the caller thread.
Other middleware paths are not changed here; they need a separate audit
and shared blocking-boundary policy.

# What's Changed

- Deferred workspace context assembly with `Mono.fromCallable(...)`.
- Scheduled the blocking assembly work on `Schedulers.boundedElastic()`.
- Added a regression test that verifies workspace context reads do not
run on the caller thread.

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Co-authored-by: Joe Zou <yixian.zou@gmail.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>