Make it so when there is a cache miss exception the pulling agent can…

[BMagerMT]

Make it so when there is a cache miss exception the pulling agent can start with the oldest message. This is an alternate approach to getting the latest token from the exception as is done in #9711.

This also solves the issue demonstrated in https://github.com/BMagerMT/OrleansStreamingIssue

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull Request Overview

This PR introduces a new OldestInStreamToken to handle cache miss scenarios by allowing the pulling agent to restart from the oldest available message in the cache, rather than passing null or attempting to extract a token from the exception.

Key Changes:

• Introduced OldestInStreamToken class that represents the oldest position in a stream
• Updated cache miss handling to use OldestInStreamToken.Instance instead of null
• Added cache logic to handle OldestInStreamToken by positioning cursors at the oldest message

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/Orleans.Streaming/Core/OldestInStreamToken.cs	New token class representing the oldest position in a stream
src/Orleans.Streaming/PersistentStreams/PersistentStreamPullingAgent.cs	Updated cache miss exception handling to use OldestInStreamToken.Instance
src/Orleans.Streaming/Common/PooledCache/PooledQueueCache.cs	Added logic to position cursor at oldest message when OldestInStreamToken is provided
test/TesterInternal/OrleansRuntime/Streams/PooledQueueCacheTests.cs	Added test coverage for getting cursor at oldest entry

[ReubenBond]

@benjaminpetit PTAL