Add read support for handles with pending payloads

[ChumpChief]

This is the read portion of placeholder handles for blobs (as prototyped in #24158). It specifically adds:

1. DocumentSchema support for the new schema flag, such that these readable clients won't blow up once the future write-mode-capable clients turn the flag on
2. ContainerRuntime/BlobManager support for waiting on blobs to appear if their handles are tagged with placeholder

Getting this up now for consideration but I'm still looking into an oddity in interactions between this read-only support and the full prototype version that I need to iron out - something weird happening when the full prototype tries to upgrade the document schema. Determined the oddities were an unrelated bug.

AB#36250

[Copilot]

Copilot reviewed 17 out of 17 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (2)

packages/runtime/container-runtime/src/blobManager/blobManager.ts:365

• Consider adding a default value for the 'placeholder' parameter (e.g., 'placeholder: boolean = false') to simplify calling this function in contexts where a placeholder is not explicitly needed.

public async getBlob(blobId: string, placeholder: boolean): Promise<ArrayBufferLike> {

packages/runtime/runtime-utils/src/remoteFluidObjectHandle.ts:38

• [nitpick] It would be helpful to add inline documentation for the 'placeholder' parameter to clarify its purpose and expected use across clients.

public readonly placeholder: boolean,

[github-actions]

🔗 No broken links found! ✅

Your attention to detail is admirable.

linkcheck output

> fluid-framework-docs-site@0.0.0 ci:check-links /home/runner/work/FluidFramework/FluidFramework/docs
> start-server-and-test "npm run serve -- --no-open" 3000 check-links

1: starting server using command "npm run serve -- --no-open"
and when url "[ 'http://127.0.0.1:3000' ]" is responding with HTTP status code 200
running tests using command "npm run check-links"


> fluid-framework-docs-site@0.0.0 serve
> docusaurus serve --no-open

[SUCCESS] Serving "build" directory at: http://localhost:3000/

> fluid-framework-docs-site@0.0.0 check-links
> linkcheck http://localhost:3000 --skip-file skipped-urls.txt

Crawling...

Stats:
  163679 links
    1315 destination URLs
    1547 URLs ignored
       0 warnings
       0 errors

[jason-ha]

One of the reasons I made #24427 was so that we can list options are true | undefined and then we don't have to pollute telemetry. All of the following line additions could be removed.

See #24399 that takes advantage.