[improve][fn] Apply incremental reload to FunctionsManager (mirror connector reload）

[Dream95]

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• I searched in the issues and found nothing similar.

Motivation

Built-in connector reload was fixed in #25773 to be incremental: unchanged NARs keep the same Connector instance and only evicted connectors are closed. Built-in function reload in FunctionsManager still uses the old replace-all + close-all pattern, which has the same bug class and should be aligned with the connector implementation.

Solution

Mirror the incremental reload approach from #25773

Alternatives

No response

Anything else?

No response

Are you willing to submit a PR?

• I'm willing to submit a PR!

[Dream95]

@lhotari Maybe this could be labeled as a good first issue

[puneetdixit200]

Hello maintainers, I would like to implement incremental reload behavior for FunctionsManager. I will compare the existing connector reload fix, add tests for unchanged and removed function NARs, and update FunctionsManager to avoid unnecessary full reloads. Please assign this to me.