inherit SessionMiddleware from parent app in ui.run_with

[evnchn]

Motivation

Fixes #2578, which boils down to how ui.run_with() absolutely does not play well with parent FastAPI app's SessionMiddleware

Implementation

When using ui.run_with() with a parent FastAPI app that already has SessionMiddleware installed, NiceGUI now:

• Inherits the secret if no storage_secret is provided or if the same secret is given (fixes case where storage silently failed)
• Detects conflicts when different secrets are used with the same cookie name, and automatically renames NiceGUI's cookie to avoid silent failures, with a warning guiding the user

Progress

• I chose a meaningful title that completes the sentence: "If applied, this PR will..."
• The implementation is complete.
• If this PR addresses a security issue, it has been coordinated via the security advisory process.
• Running with a FastAPI instance is not-in-scope for the Pytest
• Docstring will be self-explanatory and serve as documentation. But it does not show in https://nicegui.io though.