Fix integration pointer aliasing during config reload

[winhowes]

Motivation

• A reload bug caused each map entry to store the address of a loop-local copy, making all integrations alias the same object and behave like the final integration in the config.
• This could cause incorrect auth/allowlist enforcement and route traffic to the wrong upstreams after a reload, so integrations must keep distinct pointers.

Description

• Stop taking the address of the loop-local copy and instead take a pointer to the slice element with integ := &cfg.Integrations[i] and store that in the map via newMap[integ.Name] = integ in reload() (app/main.go).
• Add a regression test TestReloadMultipleIntegrationsRemainDistinct in app/reload_test.go that reloads two integrations and asserts they are distinct pointers and have different destinations.

Testing

• Ran go test ./app -run TestReloadMultipleIntegrationsRemainDistinct -count=1 -v and it passed.
• Ran go test ./app -run TestReload -count=1 and it passed.

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Codex Task