Cap fastapi <0.137 and fix execution API health empty-path route

[Revanth14]

Closes: #68562

FastAPI 0.137.0's lazy router inclusion breaks api-server and dag-processor startup in two ways: the execution API health route used @router.get("") under prefix="/health", which now raises Prefix and path cannot be both empty, and
Cadwyn's versioned router generation is not yet compatible with FastAPI 0.137.0.

• Cap fastapi<0.137.0, with uv.lock synced, to restore startup while waiting for Cadwyn compatibility.
• Change the execution API health routes to explicit paths (/health, /health/ping) so they remain valid once the cap is lifted. External paths are unchanged.
• Add a regression test to prevent reintroducing an empty health route path.

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Was generative AI tooling used to co-author this PR?

• Yes (Opus 4.8)

[shahar1]

Well done!

[Revanth14]

Thanks for the review and approval @shahar1

[potiuk]

I believe Cadwyn 7.1.0 has now support for 0.137.1+ -> I think we should bring it back @Revanth14 ?

[Revanth14]

I believe Cadwyn 7.1.0 has now support for 0.137.1+ -> I think we should bring it back @Revanth14 ?

Thanks for the heads-up @potiuk. I checked Cadwyn 7.1.0 and it now declares fastapi>=0.137.1, so I agree we can remove the temporary fastapi<0.137.0 cap.

I’ll open a follow-up PR to bump Cadwyn to >=7.1.0, relax FastAPI to >=0.137.1, refresh uv.lock, and run the relevant API/versioned-router regression tests.

[Revanth14]

I believe Cadwyn 7.1.0 has now support for 0.137.1+ -> I think we should bring it back @Revanth14 ?

Opened the follow-up PR #68826

@potiuk @shahar1 : bumped Cadwyn to >=7.1.0 and FastAPI to >=0.137.1. Heads-up that the bump surfaced a second 0.137 incompatibility beyond the empty-path fix: lazy router inclusion freezes a router's dependencies at include_router() time, so the execution API's OTel trace-context dependency (injected after assembly) was being dropped. Reworked the router assembly into a build-time factory to fix it, preserving the unsafe-always / only-authenticated / never ordering. Full execution-API suite (480 tests) green, would appreciate a look when you have a moment.
Thank you.