fix goroutine leak on fetch sleep for write requests

[WRABZY]

Main problem: The fetch goroutine in some cases ends with a sleep and freezes all allocated objects in its runtime context, even if the cancelChannel was triggered.

This fix is essentially a follow-up to PR 1644. That PR fixed the goroutine leak for read requests, but the issue still persists for write requests. This fix applies the same pattern already used for read requests in PR 1644 to the write requests.

Reproduction of the most severe case:

• The Vault agent config field exit_after_auth must be false.
• Set TokenTtl in the agent auth engine to a small value (like 10s).
• For the template section in the agent config: secrets such as PKI certificates should be used, the retrieval of which is performed via a POST request with data in the request body and has a long validity period.

Observed behavior: Every time the agent renews its token, we will see growth in newly allocated memory. This growth is proportional to the size of the non-leased secrets and their quantity in the Vault agent template config section.

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