Handle deleted rollover index exclusions

[Pybsama]

Why

ElasticGraph caches the concrete indices related to a rollover template. If one
of those indices is then deleted manually, a later range-filtered search can
still include the deleted name as an exclusion in its _msearch index
expression. OpenSearch rejects that request with index_not_found_exception.

What

• Set ignore_unavailable: true only on _msearch metadata headers whose
  index expression contains exclusions. Unlike allow_no_indices, this option
  ignores a missing concrete target such as the deleted index in an exclusion
  expression, while ordinary positive-index queries retain their existing
  failure behavior.
• Add a real-datastore regression that caches two monthly rollover indices,
  deletes one, and verifies a range-filtered search still returns the document
  from the remaining index.
• Add a unit assertion covering both the exclusion header and an adjacent
  ordinary header.

Validation

• Targeted unit specs: 27 examples, 0 failures.
• GraphQL unit suite: 1,352 examples, 0 failures.
• New real-datastore regression on OpenSearch 2.19.0 (with the repository's
  mapper-size and analysis-icu plugins): 1 example, 0 failures.
• script/lint on all three changed files.
• script/type_check.
• script/spellcheck.
• bundle exec rake schema_artifacts:check.
• Direct OpenSearch 2.19.0 _msearch verification confirmed the original
  wildcard,-deleted-index expression returns a 404 sub-response with
  allow_no_indices: true and a 200 sub-response with
  ignore_unavailable: true.
• The new :no_vcr datastore regression is also included in the CI matrix for
  Elasticsearch 9.0/9.4 and OpenSearch 2.19/3.6.

Risk assessment

Low. The change adds an officially supported _msearch metadata option only
when the generated index expression contains exclusions. Ordinary positive
index queries keep their current strict behavior. Index expressions, routing,
request bodies, query grouping, and public APIs are unchanged. The integration
regression preserves a remaining live index, so it also verifies that successful
results are not swallowed.

References

Closes #589.

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