Add application summary projection callback - #28319

Description

Applications sometimes need to contribute their own projection of data into the container summary — for example, a denormalized or application-format representation of content that is otherwise stored inside a DDS, produced incrementally alongside the runtime's ordinary summarize flow.

This PR adds a small, self-contained extension point for that: an optional applicationSummaryProjection callback on LoadContainerRuntimeParams. When provided, ContainerRuntime invokes it during summarization and inserts the returned subtree alongside its own summary roots (.channels, .metadata, etc.), tracks acceptance against summary acks/nacks, and calls back into the application (onAccepted) once a generated summary is durably accepted. Unchanged content can be returned as an ISummaryHandle pointing at the previous summary so it isn't re-serialized/re-uploaded every summary cycle.

This is intentionally scoped down from the larger prototype in #28280 (which also handles detached-construction options like fullTreePolicy and GC-adoption-correlation retry logic). Only the "register callback → contribute a subtree during summarize → correlate acceptance with the right summary" capability is included here; the detached-construction pieces are left as potential future follow-up work if/when needed.

New API surface (@fluidframework/container-runtime, @legacy @beta)

• IApplicationSummaryProjection — the callback contract (key, summarize).
• IApplicationSummaryProjectionContext — passed to summarize; exposes the previously-accepted projection (if any) for incremental reuse.
• IApplicationSummaryProjectionResult — returned from summarize; the projected ISummaryTree plus an optional onAccepted callback.
• LoadContainerRuntimeParams.applicationSummaryProjection — new optional field wiring the callback into ContainerRuntime.loadRuntime/loadRuntime2.

Tests and example

• src/test/applicationSummaryProjection.spec.ts — unit tests for the controller in isolation (key validation, handle-without-previous-summary rejection, onAccepted rejection for promise-like values, pruning of stale pending generations, multi-cycle generate → submit → ack tracking).
• src/test/summaryProjectionExample/ — a documented, runnable example (projectionSample.ts + projectionExample.spec.ts + README.md) demonstrating incremental reuse across two independently-projected blobs (left/right): after an unchanged second summary, one blob is emitted as a handle while the other remains a fresh blob. The README explicitly separates "sample code" (usable close to as-is by consumers) from "test-harness code" (written only to drive/assert the sample in this repo's test suite).

Reviewer Guidance

The review process is outlined in the pull request guidelines.

• containerRuntime.ts is the highest-risk file touched (widely used, heavily tested core class). The new capability is wired in via a new public method, initializeApplicationSummaryProjection(), called once right after construction in loadRuntime2, rather than adding a new constructor parameter — this was a deliberate choice to avoid touching the already very long constructor parameter list that other packages (e.g. mixinAttributor) depend on. Feedback on whether this is the right integration seam vs. an alternative is welcome.
• Start review at src/summary/applicationSummaryProjection.ts for the core contract/controller, then src/test/summaryProjectionExample/README.md for the intended consumer-facing usage pattern.
• This PR was extracted from a larger prototype (Application projections: deterministic loading and incremental summaries #28280) at the request of the PR author to land as an independent, minimal, well-documented capability first.