fix(cli/serve): import serve handlers lazily so the CLI works without the `serve` extra (#46473)
* fix(cli/serve): import serve handlers lazily so CLI works without the `serve` extra
* fix(cli/serve): avoid module-level future annotations to keep FastAPI route introspection working
Use TYPE_CHECKING + quoted annotations on the build_server factory signature instead of
`from __future__ import annotations`, which would stringize the @app.post route handler
annotations and make FastAPI return 422 for every request (see #44256, #44620).
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Co-authored-by: Rémi Ouazan <83456801+remi-or@users.noreply.github.com>