fix: don't call SwitchContext during in-cluster reconnect

[syscod3]

Summary

• PerformContextSwitch always calls SwitchContext, which hard-rejects any call while running in-cluster (context switching is disabled there by design, per docs/configuration.md).
• /connection/retry calls PerformContextSwitch(currentContext) to reconnect after a transient disconnect — in-cluster, that path could never succeed: it always failed on "cannot switch context when running in-cluster" and surfaced the generic connection-failed screen with a misleading raw error, instead of actually retrying.
• In-cluster mode has one fixed context, so there's nothing to switch — skip SwitchContext and go straight to the connectivity test + subsystem reinit when IsInCluster().

Found live on an in-cluster Radar deployment: backend logs showed a repeating retry loop of Context switch FAILED at SwitchContext: cannot switch context when running in-cluster, while direct API calls (GET /api/contexts) kept returning 200 — the underlying k8s connectivity was fine, only the reconnect path was structurally broken.

Test plan

• go build ./...
• go test ./internal/k8s/...

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Note

Medium Risk
Touches the shared context-switch/reconnect path after subsystem teardown; wrong branching could skip a needed kubeconfig switch or leave reconnect behavior inconsistent between modes.

Overview
PerformContextSwitch no longer calls SwitchContext when IsInCluster() is true. In-cluster Radar has a single fixed context, so reconnect (e.g. /connection/retry calling PerformContextSwitch with the current context) can proceed to the connectivity test and subsystem reinit instead of failing with cannot switch context when running in-cluster.

Out-of-cluster behavior is unchanged: SwitchContext still runs with the same error handling and timing logs.

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[syscod3]

Folded into #1314 — this fix belongs with the postgres-timeline upstream contribution already in flight.