Idempotent CREATE POLICY (CAS retry on 409 version conflict)

[bfarmer67]

Summary

Make CREATE POLICY idempotent so authors can use it inside [Migration(N, journal: false)] reconciliation migrations without hitting HTTP 409 every restart after the first.

ISM policies are versioned in OpenSearch — a plain PUT to an existing policy fails with 409 version_conflict_engine_exception. Previously the dispatcher surfaced that as Failed. Now it transparently:

1. PUT /_plugins/_ism/policies/<id> (or the legacy prefix when IsmEndpointDetectStep resolved that path).
2. On 409 → GET the existing policy, read _seq_no and _primary_term from the response root (ISM surfaces these at the document root, not under _source — unusual but documented).
3. Retry the PUT with ?if_seq_no=N&if_primary_term=M query parameters.
4. On second 409 → hard fail (concurrent writer between GET and retry; the migration lock should make this rare).

No behavior change when the policy doesn't exist — the first PUT succeeds, no GET, no retry. Mirrors LockHandle.RenewLockAsync's optimistic-concurrency pattern.

Why

PR #118 (currently in review) elevates the [Migration(N, journal: false)] + APPLY POLICY reconciliation pattern as one of three first-class temporal scopes for ISM attachment (sample 9001). The natural extension a team takes after copying that sample is to put CREATE POLICY in the same journal: false migration so the policy body itself stays in sync with source-of-truth on every deploy. Without idempotent CREATE POLICY, that pattern fails on the second startup with a confusing 409. This PR closes the gap so the reconciliation pattern is symmetric across attachment AND definition.

Tests

• Integration: extended OpenSearchTemplatePolicyIntegrationTests with CreatePolicy_TwiceWithMutatedBody_SecondCallSucceedsViaCasRetry — creates a policy, mutates its description, creates again, asserts the second call succeeds and the post-condition GET shows the mutated body. Gated by the INTEGRATIONS define and runs in the nightly Testcontainers workflow.
• Unit: 350/350 existing unit tests pass on net10.0. No new unit tests for the dispatcher CAS path — the repo has no StatementDispatcher mock infrastructure (all dispatch coverage lives in the integration suite); standing up mock plumbing for one method would be net-new work with marginal incremental value over the integration test.
• dotnet format --verify-no-changes clean.

Out of scope

• Surfacing if_seq_no / if_primary_term as a generally-available primitive for other verbs. The doc author flagged this as an optional follow-on; not needed for any current verb. Defer until a second consumer materializes.
• ADR. This is a strict tightening of an existing verb's behavior (more cases succeed, none change), not an architecture decision.

Test plan

• Provider builds on net8/9/10 — clean.
• Unit suite green — 350/350 on net10.
• Format clean.
• CI green on this PR.
• Nightly integration run picks up the new CreatePolicy_TwiceWithMutatedBody_* test.

Sequencing

This branches off main. Lands cleanly after PR #118 merges (no file conflicts — different files). The PR description references samples and README sections from #118; if this lands first, those references will be slightly forward-looking until #118 follows.