Handle deleted rollover index exclusions

[Pybsama]

Why

ElasticGraph caches the concrete indices related to a rollover template. If
those indices are then deleted manually, a later search can still target a
cached concrete name or include it as an exclusion in its _msearch index
expression. OpenSearch rejects either request with
index_not_found_exception.

What

• Always set ignore_unavailable: true on _msearch metadata headers. Unlike
  allow_no_indices, this handles both a missing concrete target and a deleted
  concrete name in an exclusion expression.
• Distinguish a zero-shard response caused by cached rollover indices being
  deleted from a genuinely unconfigured index. The router now requires every
  narrowed target to be a rollover definition with known cached indices before
  treating a zero-shard response as deleted-index state.
• Add live-datastore regressions for partial deletion, an empty time set with an
  aggregation, and deleting every cached rollover index. Unit coverage retains
  the configuration error for genuinely unconfigured rollover, non-rollover,
  narrowed, and mixed-index searches.

Validation

• Current PR-base patch on OpenSearch 3.6.0: router specs 26 examples and the
  focused Apollo/GraphQL live-datastore suite 53 examples, all passing.
• The identical patch rebased onto current main (matching stable patch ID):
  full repository suite 5,341 examples, 0 failures, with 100% line and branch
  coverage; focused live-datastore suite 55 examples, 0 failures.
• GraphQL gem suite: 1,695 examples, 0 failures, 100% line and branch coverage.
• Apollo gem suite: 123 examples, 0 failures, 100% line and branch coverage.
• script/lint, script/type_check, script/spellcheck, schema artifact check,
  and git diff --check pass.
• Site generation and internal-link checks pass. The final external-link check
  is currently blocked by redirects/timeouts and changed anchors on existing
  Apollo documentation URLs; no documentation links are changed by this PR.

Risk assessment

The change sends an officially supported _msearch metadata option on every
search. The router still raises its existing configuration error unless all
narrowed targets have cached rollover state, so a cached rollover definition
cannot hide a genuinely unconfigured non-rollover or second rollover target.
Index-expression construction, request bodies, query grouping, and public APIs
are unchanged.

References

Closes #589.

[CLAassistant]

All committers have signed the CLA.

[Pybsama]

Implemented the requested changes and resolved all four inline threads on the current single-commit head 247eef3fc1e28335f148fb47cc89beaea7c1fdc6.

• Every _msearch header now sends ignore_unavailable: true.
• Zero-shard suppression now requires every narrowed target to have cached rollover state; a mixed cached-rollover/unconfigured-non-rollover regression keeps the boundary conservative.
• Added the requested empty-time aggregation and all-cached-indices-deleted live-datastore cases, retained genuine unconfigured rollover/non-rollover coverage, and removed :no_vcr.

Local verification on OpenSearch 3.6.0 passes the focused live-datastore suite and router specs. The identical patch on current main passes 5,341 repository examples with 100% line and branch coverage, plus lint, type, spelling, and schema checks. Site generation and internal links pass; only unchanged Apollo documentation external URLs fail the final external-link probe because of redirects/timeouts and changed anchors.

Marking this ready for re-review.