runtime: Add "retired" features implementation for Document Schema

[scottn12]

Description

Adds a "retired" feature mechanism to Document Schema. A retired feature is one that was previously toggleable via IDocumentSchemaFeatures but is now hardcoded to a single value by the runtime.

Why we need this: existing documents may still carry a retired feature's key in their persisted
schema, so Document Schema must continue to recognize it (otherwise checkRuntimeCompatibility
would fail the session on an "unknown" runtime property). At the same time, we no longer want
that feature to be part of the public IDocumentSchemaFeatures surface, because callers can
no longer influence it.

Retired features are tracked in a separate retiredDocumentSchemaFeatures map. Each entry
bundles the property handler with the runtime's hardcoded value. They participate in the
normal validate / merge / schema-change-op flow exactly like non-retired features — the only
difference is that the value comes from the map instead of from the constructor's features
argument. A compile-time satisfies constraint ensures a key cannot appear in both
IDocumentSchemaFeatures and retiredDocumentSchemaFeatures.

Note: No features are retired yet. This PR just sets up the infrastructure for when features are retired in the future.

Misc

AB#72041

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Pull request overview

Adds infrastructure to support "retired" Document Schema features — features previously toggleable via IDocumentSchemaFeatures but now hardcoded by the runtime. Persisted documents may still carry these keys, so they must remain recognized during compatibility checks and merging, even though they're no longer part of the public surface. No features are retired yet; this PR only sets up the mechanism.

Changes:

• Introduces retiredDocumentSchemaFeatures map (currently empty) with a compile-time satisfies guard ensuring keys cannot overlap with IDocumentSchemaFeatures.
• Adds getRuntimeConfigHandler to resolve a property handler from either supported or retired configs, used in checkRuntimeCompatibility, and, and or.
• Adds retiredFeatureValues helper, spread into the runtime portion of the desired schema in DocumentsSchemaController.