Retry throttled ARM Machine requests

[Wenxuan W (wenxuan0923)]

Summary

• configure the shared ARM Machine SDK pipeline with a bounded 30-retry budget for every Machine request
• honor ARM-provided Retry-After, Retry-After-Ms, and x-ms-retry-after-ms delays before using jittered exponential backoff
• apply the same retry behavior to direct ARM and dev-proxy Machine clients
• cover GET, create/update, retry-delay headers, and exhausted retry budgets

Validation

• make check
• go test -race ./pkg/aksmachine

Fixes #285

[Copilot]

Pull request overview

This PR hardens the AKS ARM Machine client against transient ARM throttling (HTTP 429) during bootstrap by configuring the Azure SDK retry policy to use a larger, bounded retry budget and to respect ARM-provided retry-delay headers, addressing the failure described in issue #285.

Changes:

• Add a dedicated ARM Machine SDK retry configuration (30 retries, 2m per-try timeout, 5s base retry delay, 1m max backoff) and apply it to the standard ARM Machine client.
• Apply the same retry configuration to the dev-proxy ARM Machine client so proxy usage matches production retry behavior.
• Add targeted tests covering option wiring, retry on 429 for GET and create/update, honoring retry-delay headers, and stopping after the retry budget is exhausted.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
pkg/aksmachine/client_armapi.go	Introduces and applies a shared ARM Machine-specific Azure SDK retry configuration.
pkg/aksmachine/client_armapi_test.go	Adds tests validating retry configuration and behavior for throttled ARM Machine requests.
pkg/aksmachine/client_armapi_proxy.go	Reuses the same ARM Machine retry configuration for the dev-proxy client.

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