GO-7116 Fix deadlock on app close in inboxclient

[requilence]

Summary

• Add componentCtx to inboxclient that gets cancelled on Close(), ensuring all operations (both periodic and external callback) are cancellable on shutdown
• Propagate context through fetchMessages/getOffset/setOffset instead of hardcoded context.Background()

Problem

inboxclient.Close() deadlocks when the coordinator connection is stale. The periodic sync callback (checkMessages) ignores its context parameter and calls fetchMessages() which uses context.Background() for the InboxFetch RPC. When the coordinator doesn't respond, the RPC blocks forever, and periodicCall.Close() waits indefinitely for the loop to finish.

Deadlock chain:

1. app.Close() → inboxclient.Close() → periodicCheck.Close() → cancels loop context, waits on <-loopDone
2. Periodic loop executing checkMessages(ctx) → ignores ctx → fetchMessages() → InboxFetch(context.Background()) → blocked forever on stale DRPC stream

Test plan

• All existing inboxclient tests pass
• ReceiveNotify (external callback) now uses componentCtx instead of context.Background() — also cancellable on close

[github-actions]

New Coverage	47.2% of statements
Patch Coverage	47.6% of changed statements (20/42)

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