fix(server): FastAPI 0.137 compatibility — skip route deletion when router is referenced

[devin-ai-integration]

closes #22276

FastAPI 0.137 changed include_router to wrap the original router in an _IncludedRouter that references it for request matching, instead of copying routes into the parent. The existing memory optimisation (del router.routes after inclusion) deletes the routes that _IncludedRouter still needs, causing AttributeError: 'PrefectRouter' object has no attribute 'routes' on every request.

The fix gates the deletion behind a version check:

_FASTAPI_COPIES_ROUTES_ON_INCLUDE = Version(
    importlib.metadata.version("fastapi")
) < Version("0.137.0")

# in create_api_app:
if final and _FASTAPI_COPIES_ROUTES_ON_INCLUDE:
    del router.routes

• FastAPI < 0.137: routes are still deleted after inclusion (memory savings preserved)
• FastAPI ≥ 0.137: routes are kept — no duplication occurs anyway since the app references the original router directly

Checklist

• This pull request references any related issue by including "closes <link to issue>"
• If this pull request adds new functionality, it includes unit tests that cover the changes
• If this pull request removes docs files, it includes redirect settings in mint.json.
• If this pull request adds functions or classes, it includes helpful docstrings.

Link to Devin session: https://app.devin.ai/sessions/cc191d244cbf43b48183c85d17dc8870

[devin-ai-integration]

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[codspeed-hq]

Merging this PR will not alter performance

✅ 8 untouched benchmarks
⏩ 1 skipped benchmark¹

---

_{Comparing devin/1781459497-fastapi-0137-compat (a75209e) with main (e948a74)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩

[desertaxle]

Could we make this check version-based rather than relying on the presence of a private class?

[devin-ai-integration]

Good call — switched to packaging.version.Version(importlib.metadata.version("fastapi")) < Version("0.137.0") in a75209e.