Harden Cost Management export retries for HTTP 429

🐛 Problem

Start-FinOpsCostExport can fail instead of retrying when the Cost Management Run API returns HTTP 429. Invoke-Rest emits Write-Error for the response before the caller can reliably inspect Throttled, and the loop condition does not repeat a single-export run even when throttling is returned non-terminatingly.

The current unit test mocks Invoke-Rest returning a throttled response directly, so it does not cover the terminating error behavior observed in CI.

👣 Repro steps

1. Run New-FinOpsCostExport -Execute or Start-FinOpsCostExport in a context where PowerShell treats Write-Error as terminating.
2. Have the Cost Management export Run API return HTTP 429 with a Retry-After value.
3. Observe that the command exits with an error instead of waiting and retrying.

🤔 Expected

• HTTP 429 responses are returned to Start-FinOpsCostExport in a form it can handle.
• The command honors the server-provided Retry-After value, with a safe fallback when absent or invalid.
• Both single-export and multi-month runs retry the same request after throttling.
• Retries are bounded to prevent an infinite loop.
• Unit tests reproduce the real Invoke-Rest/Write-Error behavior and cover single-export, multi-month, retry-delay, retry-limit, and eventual-success cases.

📷 Screenshots

N/A

🔧 Environment

• FinOps hub version: Development branch / PR #2187
• Billing account type: N/A
• Power BI report type: Storage-based deployment
• Cost Management export: FOCUS cost

ℹ️ Additional context

Observed in the Hubs (manual) check for PR #2187: deployment completed, then the initial export run failed with Too many requests. Please retry after 60 seconds. (Code: 429).

Failed run: https://github.com/microsoft/finops-toolkit/actions/runs/31678558903

The two immediately preceding PR deployment runs succeeded, indicating the service-side 429 was transient while the retry-handling failure is deterministic.

🙋‍♀️ Ask for the community

We could use your help:

1. Please vote this issue up (👍) to prioritize it.
2. Share other Cost Management throttling responses, especially variations in status codes or retry headers.

Start with Start-FinOpsCostExport and its Invoke-Rest call, then inspect the existing unit test that mocks a throttled response directly. Reproduce the terminating Write-Error behavior and cover single-export, multi-month, delay, retry-limit, and eventual-success cases. Done means HTTP 429 responses are retried with Retry-After handling and bounded retries.