Fix staticcheck lint warnings

[JamieMagee]

Fixes 18 of the 24 staticcheck warnings reported by golangci-lint run --enable-only staticcheck.

What changed:

• S1034 — config.GetListOfStrings now uses a tagged type switch (switch val := value.(type)) instead of switching on the type and then repeating the assertion inside each case.
• QF1001 — Replaced !(a || b) with !a && !b (De Morgan's law) in the dialer and four handler guard clauses. Same logic, easier to read.
• QF1003 — Swapped an if-else chain for a switch on metricType in the metrics collector client.
• SA4006 — Discarded unused return values (_, _) in docker registry and cache tests where the returned req/resp was immediately overwritten.
• SA5011 — Added an explicit return after t.Fatalf in the OIDC credential test so staticcheck can prove actual is non-nil past that point.

Not included:

The remaining 6 warnings are SA1019 (deprecated aws-sdk-go v1 imports in the docker registry handler). Migrating to aws-sdk-go-v2 changes the API surface and deserves its own PR.

[Copilot]

Pull request overview

This PR reduces staticcheck noise across the proxy codebase by applying small refactors and test cleanups that preserve existing behavior while eliminating flagged patterns.

Changes:

• Refactors type checks / conditionals (tagged type switch, De Morgan rewrite, switch on metric type) to satisfy staticcheck quick-fix recommendations.
• Cleans up tests by discarding unused return values and adding an explicit return after t.Fatalf so staticcheck can prove control flow.
• Minor readability improvements in request guard clauses and dialer network-type validation.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
internal/oidc/oidc_credential_test.go	Adds an explicit return after t.Fatalf to satisfy SA5011 flow analysis.
internal/metrics/collector_client.go	Replaces an if/else chain with a switch when populating metric payload fields (QF1003).
internal/handlers/python_index.go	Applies De Morgan rewrite in handler credential matching guard (QF1001).
internal/handlers/nuget_feed.go	Applies De Morgan rewrite in handler credential matching guard (QF1001).
internal/handlers/maven_repository.go	Applies De Morgan rewrite in handler credential matching guard (QF1001).
internal/handlers/goproxy_server_handler.go	Applies De Morgan rewrite in handler credential matching guard (QF1001).
internal/handlers/docker_registry_test.go	Discards unused returned req values in tests where they are overwritten (SA4006).
internal/dialer/dialer.go	Rewrites negated disjunction to conjunction for readability (QF1001).
internal/config/config.go	Uses a tagged type switch to avoid repeated type assertions (S1034).
internal/cache/handlers_test.go	Avoids assigning unused return value from OnResponse (SA4006).

Comments suppressed due to low confidence (1)

internal/config/config.go:71

• In GetListOfStrings, the local variable named strings shadows the imported strings package in this file, which makes the code harder to read and can lead to confusion if additional strings.* calls are added later. Consider renaming the slice to something like result/out/list.

		strings := make([]string, len(val))
		for i, v := range val {
			if str, ok := v.(string); ok {
				strings[i] = str
			}
		}
		return strings