[ES|QL][Inference] Add Cross-Cluster Search (CCS) support for all inference commands

[afoucret]

Problem

The goal of this issue is to ensure that ES|QL inference commands work correctly in cross-cluster search (CCS) scenarios.

Following team discussions, we are simplifying the approach:

• Inference always executes on the coordinator cluster receiving the query.
• No remote execution of inference is required, and qualifiers (_coordinator, _remote) are no longer needed.
• TEXT_EMBEDDING continues to execute on the coordinator as part of the pre-optimizer phase.

Acceptance criteria

• Commands and function fail early if the inference endpoint does not exist on the coordinator.
• RERANK command supports CCS searches
• COMPLETION command supports CCS searches
• TEXT_EMBEDDING function supports CCS searches
• Existing CSV tests run using multi-clusters

Proposed solution

RERANK and COMPLETION:

• Execute on the coordinator cluster.
• Validate that the inference endpoint exists on the coordinator; fail if missing.
• Remove any checks that previously blocked inference for remote indices.

TEXT_EMBEDDING:

• Execution remains on the coordinator during the pre-optimizer phase.
• No change is required for CCS

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[elasticsearchmachine]

Pinging @elastic/search-relevance (Team:Search - Relevance)