[azcosmos] Control-plane retry: cross-region failover preempted by caller ctx cancellation

## Summary Same class of defect as Azure/azure-cosmos-dotnet-v3#5805 (fix in Azure/azure-cosmos-dotnet-v3#5806). `cosmos_client_retry_policy.go` honors the caller's `ctx.Err()` and exits the retry loop before consulting the cross-region retry decision on control-plane reads. When an unhealthy preferred region triggers internal HTTP timeout escalation, a caller whose `ctx` deadline fires during escalation never gets the cross-region attempt that the SDK advertises. The global endpoint manager policy (`cosmos_global_endpoint_manager_policy.go:22-24`) already uses `context.WithoutCancel` to shield its own internal retries from caller cancellation — the same pattern is missing from the client retry policy for control-plane reads. Note: azcosmos has no collection / partition-key-range cache today, so the exact customer-visible failure mode of the .NET bug does not reproduce. This issue is filed to: 1. Fix the existing gap on `GetAccountProperties` and region discovery, which are affected today. 2. Prevent the defect class from silently recurring when / if a collection or partition cache is later added (at which point control-plane reads gate data-plane traffic, as in .NET / Python). ## Files / lines - `sdk/data/azcosmos/cosmos_client_retry_policy.go` — client retry policy (~lines 24-79) - `sdk/data/azcosmos/cosmos_global_endpoint_manager_policy.go:22-24` — reference for the `context.WithoutCancel` pattern already used locally. ## Repro (sketch) 1. Fresh `cosmos.Client`. 2. Preferred regions `[A, B]`; `A` unreachable (blackhole / 503). 3. Call `client.AccountProperties(...)` with a context bounded by `context.WithTimeout(ctx, 36*time.Second)`. 4. Observe: context deadline fires, no attempt against region `B` is made. ## Expected behavior When the caller's `ctx` is cancelled during an in-flight control-plane attempt, the retry policy is consulted; if it indicates a cross-region retry, one bounded attempt against the next region executes on a retry-scoped context derived with `context.WithoutCancel` + `context.WithTimeout(grace)`. ## Proposed fix direction ```go retryCtx, cancel := context.WithTimeout(context.WithoutCancel(ctx), graceWindow) defer cancel() // attempt against next region using retryCtx ``` Surface the original `ctx.Err()`-bearing error as the cause if the grace attempt also fails or expires. Grace window of 10s aligns with the .NET fix. ## Cross-references This defect class was identified in a cross-SDK investigation prompted by the .NET fix. Tracking issues: - .NET (fix landed): Azure/azure-cosmos-dotnet-v3#5805 — PR Azure/azure-cosmos-dotnet-v3#5806 - Python: Azure/azure-sdk-for-python#46471 - Rust: Azure/azure-sdk-for-rust#4253 - Java: not filed — Reactor's `retryWhen` operator structurally isolates subscription cancellation from the retry decision, so the defect does not reproduce. /cc @NaluTripician