Avoid snapshot in ALTER SINK if the sink has made progress (#34588)

In the case of an `ALTER SINK`, don't snapshot if the sink upper has
progressed beyond the as_of.

### Motivation

fixes: MaterializeInc/database-issues#9998

### Checklist

- [ ] This PR has adequate test coverage / QA involvement has been duly
considered. ([trigger-ci for additional test/nightly
runs](https://trigger-ci.dev.materialize.com/))
- [ ] This PR has an associated up-to-date [design
doc](https://github.com/MaterializeInc/materialize/blob/main/doc/developer/design/README.md),
is a design doc
([template](https://github.com/MaterializeInc/materialize/blob/main/doc/developer/design/00000000_template.md)),
or is sufficiently small to not require a design.
  <!-- Reference the design in the description. -->
- [ ] If this PR evolves [an existing `$T ⇔ Proto$T`
mapping](https://github.com/MaterializeInc/materialize/blob/main/doc/developer/command-and-response-binary-encoding.md)
(possibly in a backwards-incompatible way), then it is tagged with a
`T-proto` label.
- [ ] If this PR will require changes to cloud orchestration or tests,
there is a companion cloud PR to account for those changes that is
tagged with the release-blocker label
([example](MaterializeInc/cloud#5021)).
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- [ ] If this PR includes major [user-facing behavior
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I have pinged the relevant PM to schedule a changelog post.

Author: martykulma

src/storage-controller/src/lib.rs

@@ -1577,6 +1577,15 @@ where
         };
         *cur_export = new_export;
 
+        // For `ALTER SINK`, the snapshot should only occur if the sink has not made any progress.
+        // This prevents unnecessary decoding in the sink.
+        // If the write frontier of the sink is strictly larger than its read hold, it must have at
+        // least written out its snapshot, and we can skip reading it; otherwise assume we may have
+        // to replay from the beginning.
+        // TODO(database-issues#10002): unify this with run_export, if possible
+        let with_snapshot = new_description.sink.with_snapshot
+            && !PartialOrder::less_than(&new_description.sink.as_of, &cur_export.write_frontier);
+
         let cmd = RunSinkCommand {
             id,
             description: StorageSinkDesc {
@@ -1587,7 +1596,7 @@ where
                 as_of: new_description.sink.as_of,
                 version: new_description.sink.version,
                 from_storage_metadata,
-                with_snapshot: new_description.sink.with_snapshot,
+                with_snapshot,
                 to_storage_metadata,
                 commit_interval: new_description.sink.commit_interval,
             },