fix(oauth): reuse AccessToken API client to fix unbounded clientMap memory leak

[DBGR18]

Summary:
oauth.sendAccTokenReq caches the AccessToken.APIClient in a package-level clientMap sync.Map, but keys the cache by the freshly-allocated *Configuration returned from NewConfiguration() on every call. Because that pointer is different on each invocation, the cache lookup never hits and a new entry (each holding an *AccessToken.APIClient with its own http.Client) is stored on every token request. The map only ever grows.

This is the root cause of free5gc/free5gc#1060.

Root cause:
oauth/get_token_context.go:

var clientMap sync.Map // package-level, never pruned

func sendAccTokenReq(...) (...) {
    configuration := AccessToken.NewConfiguration() // new *Configuration every call
    configuration.SetBasePath(nrfUri)

    if val, ok := clientMap.Load(configuration); ok { // key = ephemeral pointer -> always miss
        client = val.(*AccessToken.APIClient)
    } else {
        client = AccessToken.NewAPIClient(configuration)
        clientMap.Store(configuration, client) // new permanent entry on every request
    }
    ...
}

NewConfiguration() returns a *Configuration, so the map key is a distinct pointer on every call. tokenMap (keyed by the scope string) is fine and is correctly reused; only clientMap leaks.

Fix:
Key clientMap by the stable nrfUri string so the client is actually reused:

if val, ok := clientMap.Load(nrfUri); ok {
    client = val.(*AccessToken.APIClient)
} else {
    configuration := AccessToken.NewConfiguration()
    configuration.SetBasePath(nrfUri)
    client = AccessToken.NewAPIClient(configuration)
    clientMap.Store(nrfUri, client)
}

[Copilot]

Pull request overview

This PR fixes an unbounded memory leak in the OAuth AccessToken client cache by ensuring the cache key is stable across calls, so *AccessToken.APIClient instances are actually reused rather than continuously accumulated.

Changes:

• Re-key clientMap from ephemeral *Configuration pointers to the stable nrfUri string to enable cache hits.
• Move NewConfiguration() creation into the cache-miss path to avoid unnecessary allocations.
• Add an explanatory comment documenting the original leak mechanism and the rationale for the fix.

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