KUBEDR-7763: Don't panic if CanShareWork() is called on a closed workpool

[dzaninovic]

Don't panic if CanShareWork() is called on a closed workpool
Log a warning instead.
Context passing is needed to make the log levels work.
I tested to make sure the log is printed in our mover log.

[Copilot]

Pull request overview

This PR updates the internal/workshare worker pool to avoid panicking when AsyncGroup.CanShareWork() is called after the pool has been closed, and instead emits a warning log (requiring context to be passed into pool construction so logging can use the correct context-derived log level behavior).

Changes:

• Change workshare.NewPool to accept a context.Context and store it on the pool.
• Update AsyncGroup.CanShareWork() to return false (and warn) instead of panicking when the pool is closed.
• Update snapshotfs call sites and workshare tests to pass context into NewPool, and adjust expectations for the new CanShareWork() behavior.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
snapshot/snapshotfs/upload.go	Passes ctx into workshare.NewPool when creating the uploader worker pool.
snapshot/snapshotfs/snapshot_tree_walker.go	Passes ctx into workshare.NewPool for TreeWalker parallel traversal.
snapshot/snapshotfs/dir_rewriter.go	Passes ctx into workshare.NewPool for directory rewriting work sharing.
internal/workshare/workshare_waitgroup.go	Replaces CanShareWork() panic-on-closed with warning + false return.
internal/workshare/workshare_test.go	Updates tests/benchmarks to use the new NewPool(ctx, n) signature and new CanShareWork() behavior.
internal/workshare/workshare_pool.go	Adds context storage to Pool and updates NewPool signature accordingly.

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[dzaninovic]

Kopia linter did not like me passing context in a struct. I will fix that.

/home/davidz/cloudcasa/kopia/tools/.tools/golangci-lint-1.62.0/golangci-lint --timeout 1200s run 
internal/workshare/workshare_pool.go:25:2: found a struct that contains a context.Context field (containedctx)
	ctx       context.Context
	^

[dzaninovic]

We will fix the linter issue later.