Fix detector creation with aliases and data streams

[NelTdS]

Description

Fixes several bugs that prevented detector creation when the configured index is an index pattern, alias, or data stream.

Six root causes addressed:

1. GetIndexMappingsRequest null index (TransportIndexDetectorAction): IndexUtils.getNewIndexByCreationDate() returns null when no concrete index matches the pattern. The null was passed directly to GetIndexMappingsRequest, triggering Validation Failed: 1: index_name is missing. Fix: resolve pattern to concrete index first; fall back to the original name when resolution returns null.
2. Strict index existence check (TransportIndexDetectorAction): The pre-creation SearchRequest used default strict IndicesOptions, producing Indices not found for patterns with no current backing indices. Fix: use IndicesOptions.LENIENT_EXPAND_OPEN.
3. GetMappingsRequest strict resolution (MapperService): Three GetMappingsRequest calls used default options, causing OpenSearch to search for a template named -template when the pattern had no backing index, producing a misleading 404. Fix: add LENIENT_EXPAND_OPEN to all three calls.
4. GetIndexRequest strict resolution (MapperService): resolveConcreteIndex() used a strict GetIndexRequest, failing with no such index for patterns with no current backing index. Fix: use LENIENT_EXPAND_OPEN; return the original name on empty result.
5. NoSuchElementException on empty mappings (MapperService): doGetMappingAction and doGetMappingsView called .iterator().next() on the mappings map without an emptiness check. A data stream with no mappings returned an empty map, causing an uncaught exception. Fix: guard both call sites; return an empty response instead.
6. Race condition on concurrent detector creation (TransportIndexDetectorAction): When multiple detectors are created in parallel, all requests read cluster state simultaneously, see the detectors index absent, and all issue CreateIndexRequest. The first succeeds; the remaining requests received ResourceAlreadyExistsException and failed fatally. Fix: catch ResourceAlreadyExistsException in the onFailure handler of initDetectorIndex and treat it as success.

Related Issues

None

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.