feat: Add PUT endpoint for flows upsert (#11368)

* feat: Implement upsert functionality for flows with conflict handling and validation

* test: add tests for upsert

* [autofix.ci] apply automated fixes

* fix: handle None data in webhook check

* fix: allow clearing endpoint_name on PUT update

* fix: validate empty fs_path on update and assert login success in test

* refactor: address review feedback - improve docstrings, use 404 for security, consistent JSONResponse

* refactor: move persistence logic into _new_flow and simplify return pattern

_new_flow now handles flush, refresh, and filesystem save internally,
returning FlowRead directly. This eliminates duplication between POST
and PUT endpoints. PUT endpoint now uses single return with variable
status code.

* fix: update upload endpoint to work with refactored _new_flow

_new_flow now handles persistence and returns FlowRead directly,
so the upload endpoint no longer needs to call flush/refresh/save.

* [autofix.ci] apply automated fixes

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Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>