feat(indexing): support typed virtual stream routing

[yordis]

• Indexing-owned virtual streams need a typed routing boundary before concrete readers can safely share the read pipeline.
• Centralizing stream matching keeps future reader implementations focused on indexing behavior instead of repeating identifier checks.

[cursor]

PR Summary

Low Risk
New abstract type and tests only; no changes to storage read paths or node wiring in this diff.

Overview
Introduces an abstract IndexVirtualStreamReader that implements IVirtualStreamReader for indexing-owned virtual streams. Each reader is bound to a typed IndexStreamId at construction (null rejected), exposes that id, and implements CanReadStream by comparing the candidate string to StreamId.Value so routing stays in one place. Forward/backward reads and last event/index position remain abstract for concrete indexing readers.

Adds IndexVirtualStreamReaderTests with a minimal test subclass covering constructor validation, StreamId exposure, and match/mismatch/null CanReadStream behavior.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 0274f6b4-3851-4e78-94e7-4c2a1981f958

📥 Commits

Reviewing files that changed from the base of the PR and between 0793925 and 96ea9c9.

📒 Files selected for processing (2)

• src/EventStore.Core.XUnit.Tests/Services/Storage/Indexing/IndexVirtualStreamReaderTests.cs
• src/EventStore.Core/Services/Storage/Indexing/IndexVirtualStreamReader.cs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch yordis/feat-index-virtual-reader

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