Cancel sidecar parent-block fetches on sync service shutdown

[syjn99]

What type of PR is this?

Bug fix

What does this PR do? Why is it needed?

When a blob or data column sidecar arrives over gossip before its parent block, the (blob/data column) validator spawns a goroutine that requests the parent by root (sendBatchRootRequest).
Since #13061 that goroutine ran on context.Background(), so it never observed beacon node shutdown, and a cancelled context was not honored mid-request either.

This PR:

• Runs the sidecar parent fetch on the sync service context (s.ctx) instead of context.Background().
• Registers context.AfterFunc(ctx, stream.Reset) in SendBeaconBlocksByRootRequest and SendExecutionPayloadEnvelopesByRootRequest, so cancellation interrupts a blocked response read instead of waiting for the next chunk deadline.
• Checks ctx.Err() at the top of each retry and after each attempt in sendBatchRootRequest, so a cancelled request returns immediately instead of burning the remaining numOfTries attempts.
• Bounds the optimistic payload envelope fetch with respTimeout per attempt, the same bound sendBeaconBlocksRequest already applies to the block request. Previously the envelope request inherited the caller's context with no timeout of its own.

Which issue(s) does this PR fix?

N/A

Other notes for review

Acknowledgements

• I have read CONTRIBUTING.md.
• I have included a uniquely named changelog fragment file.
• I have added a description with sufficient context for reviewers to understand this PR.
• I have tested that my changes work as expected and I added a testing plan to the PR description (if applicable).