fix(downtime_schedules): reconcile destination conflicts

[michael-richey]

Summary

Handle deterministic downtime destination conflicts without leaving stale or unmanaged state.

What this PR changes

• On a duplicate-create 400, parse the structured API error. If it identifies exactly one existing downtime, fetch that resource and persist its source-to-destination mapping.
• If a duplicate response has no candidate ID or multiple candidate IDs, preserve the original error so normal failure accounting remains visible.
• On an update 404 with the specific Downtime not found error, recreate the downtime and replace the stale destination mapping with the new response.
• Propagate unrelated update 404 responses.
• Treat delete 404 as an idempotent success.

Test plan

• Duplicate-create reconciliation persists the existing destination resource.
• Missing and ambiguous duplicate IDs propagate.
• Update-not-found recreation replaces stale state and omits the obsolete destination ID.
• Unrelated update 404 and other 4xx/5xx responses propagate.
• Delete 404 remains idempotent.
• Focused downtime tests pass.
• Non-subprocess unit suite passes.
• Ruff and Black checks pass.

[Copilot]

Pull request overview

This PR improves downtime_schedules sync robustness by handling deterministic destination-side conflict and “gone” conditions in a way that avoids leaving stale/unmanaged destination mappings, while still propagating unrelated failures for normal retry/accounting.

Changes:

• Reconcile create-time duplicate (400) by parsing the structured error body, fetching the single unambiguous existing downtime, and persisting its mapping.
• Handle update-time not-found (404 with “Downtime not found”) by recreating the downtime and replacing the stale destination mapping; propagate unrelated 404s.
• Expose HTTP response bodies on CustomClientHTTPError to enable structured error parsing, and add focused unit tests for the new behaviors (including delete 404 idempotency).

Reviewed changes

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

File	Description
datadog_sync/model/downtime_schedules.py	Adds deterministic conflict/not-found handling for create/update/delete and factors create-schedule normalization into a helper.
datadog_sync/utils/resource_utils.py	Extends CustomClientHTTPError to retain the response body for downstream error inspection.
tests/unit/test_downtime_schedules_conflict_skip.py	Adds unit coverage for duplicate-create reconciliation, update-not-found recreation, and delete 404 idempotency.

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