feat(framework): Add opt-in runtime version warnings for ServerAppIo

[mohammadnaseri]

This PR stops the ServerAppIo runtime version interceptor from returning incompatibility warning metadata by default.
The interceptor still parses and checks runtime version metadata, and the client-side warning handling remains in place for future opt-in warnings.

[chatgpt-codex-connector]

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Pull request overview

This PR makes runtime version incompatibility warnings for the ServerAppIo gRPC API opt-in by defaulting the server-side runtime version interceptor to observe-only (i.e., it still parses/checks metadata but does not attach incompatibility warning metadata unless explicitly enabled).

Changes:

• Introduced RuntimeVersionCompatibilityAction to control server-side behavior on incompatible/unparseable runtime version metadata.
• Updated RuntimeVersionServerInterceptor and create_serverappio_runtime_version_server_interceptor to default to OBSERVE_ONLY and only emit warning trailing metadata when configured with WARN.
• Adjusted and extended unit tests to verify default non-warning behavior and explicit warning behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
framework/py/flwr/supercore/interceptors/runtime_version_interceptor.py	Adds compatibility action enum and gates trailing warning metadata emission behind an explicit WARN action.
framework/py/flwr/supercore/interceptors/runtime_version_interceptor_test.py	Updates tests to assert no trailing metadata by default and adds coverage for explicit warning mode.
framework/py/flwr/supercore/interceptors/__init__.py	Exposes RuntimeVersionCompatibilityAction via the interceptors package exports.

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[panh99]

LGTM!