fix(indexing): validate index definitions

[yordis]

• Prevent invalid index definitions from entering the indexing lifecycle before transport wiring exists.
• Keep empty selectors and empty definitions from becoming delayed runtime failures.

[cursor]

PR Summary

Low Risk
New domain types and tests only; no wiring to storage or transport yet, so runtime behavior is unchanged.

Overview
Introduces index definition model types (IndexDefinition, IndexEventFilter, IndexFieldDefinition, IndexFieldSelector) under EventStore.Core.Services.Storage.Indexing, with fail-fast validation when they are constructed.

IndexDefinition requires a non-null fields list, rejects null entries, and refuses a definition with no filter and no fields; it defensively copies the field list into an array. IndexEventFilter, field names, and selectors reject null/whitespace strings; field definitions require a non-null selector.

Adds xUnit coverage in IndexDefinitionTests for these validation rules and for allowed shapes (filter-only, field-only, and field-list immutability).

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 1870fcd8-f81f-4dc5-b852-5715520cf6e8

📥 Commits

Reviewing files that changed from the base of the PR and between 8e4152f and af8afd5.

📒 Files selected for processing (2)

• src/EventStore.Core.XUnit.Tests/Services/Storage/Indexing/IndexDefinitionTests.cs
• src/EventStore.Core/Services/Storage/Indexing/IndexDefinition.cs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch yordis/fix-index-definition-validation

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