Closes
[CODY-2775](https://linear.app/sourcegraph/issue/CODY-2775/%5Bautocomplete-latency%5D-apply-the-same-timeout-on-the-cody-gateway-side)



Enables client control over the request processing timeout on the server
(both Sourcegraph backend and Cody Gateway). The context timeout is set
to the value provided in the `X-Timeout-Ms` header of the client
request. If the header is not provided, the default context timeout is
used (1 minute on both Sourcegraph backend and Cody Gateway).

Previously, we only had a default timeout on the Sourcegraph backend
side (8 minutes).

Corresponding client change:
- https://github.com/sourcegraph/cody/pull/4921

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## Test plan
- Manually tested and confirmed that if the request contains the
`X-Timeout-Ms` header, its value is used. If not, the default maximum
request duration is applied.
- CI
- 
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## Changelog
- Use the provided timeout from request parameters if available;
otherwise use the default maximum request duration (8 minutes)

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