fix(silo): Reduce timeout and disable retries for synchronous proxy requests

[sentry]

This PR addresses a "Blocking Operation" performance issue (SENTRY-5PX9) observed on the /api/0/organizations/{organization_id_or_slug}/issues/ endpoint.

Problem:
The ratelimit.__call__ middleware was blocking for ~2.4 seconds due to a synchronous HTTP call made by CellSiloClient.proxy_request() to an internal Cell Silo endpoint. This blocking was exacerbated by a long default timeout and inappropriate retry logic for synchronous proxy operations.

Changes:

1. Reduced CellSiloClient timeout: The default timeout for CellSiloClient has been reduced from 30 seconds to 10 seconds. This prevents a single unresponsive Cell Silo from blocking a Control Silo request thread for an excessively long duration.
2. Disabled retries for proxy_request: The proxy_request method now explicitly disables retries before making the super()._request() call. Synchronous proxy requests within a Django request cycle should fail fast rather than retrying with backoff, which would compound the blocking time and worsen user experience.

These changes ensure that synchronous cross-silo proxy calls are more resilient to slow Cell Silo responses and do not excessively block the Control Silo's request processing.

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Fixes SENTRY-5PX9

[github-actions]

Backend Test Failures

Failures on ac98529 in this run:

tests/sentry/middleware/integrations/parsers/test_jira.py::JiraRequestParserTest::test_get_response_routing_to_cell_sync_retry_errors — log

[gw0] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/middleware/integrations/parsers/test_jira.py:117: in test_get_response_routing_to_cell_sync_retry_errors
    assert len(responses.calls) == 6
E   assert 1 == 6
E    +  where 1 = len(<responses.CallList object at 0x7f92a145f0e0>)
E    +    where <responses.CallList object at 0x7f92a145f0e0> = responses.calls