fix(preload): Prevent [null,null] unhandled rejections from preload-data.html

[sentry]

This PR addresses the escalating volume of [null,null] unhandled rejection events originating from the inline preload script in src/sentry/templates/sentry/partial/preload-data.html.

Problem:
The promiseRequest function within preload-data.html was designed to throw [response.status, response.statusText] for non-2xx responses and throw [error.status, error.statusText] for network failures. When a preload fetch received no response, error.status and error.statusText were undefined, leading to a rejection with [undefined, undefined], which the SDK normalized to [null,null] via onunhandledrejection.

Root Cause:
PR #120588 (merged 2026-07-24) fixed the sibling bundled copy (static/app/bootstrap/index.tsx) to return null in both error branches and, crucially, removed the isNullTupleUnhandledRejectionEvent beforeSend filter that previously suppressed these events. The preload-data.html template was never updated, so it continued to throw and, with the filter removed, these events began to be reported to Sentry.

Solution:
This change mirrors the fix implemented in PR #120588. The promiseRequest function in src/sentry/templates/sentry/partial/preload-data.html is updated to return null instead of throw in both the non-2xx response handling and the catch block. This ensures that failed preload requests are handled gracefully without generating unhandled rejections.

Impact:
The shared consumer getPreloadedData in static/app/bootstrap/bootstrapRequests.tsx already expects and handles null returns, falling back to a normal fetch. This change has a low blast radius, aligns the template's behavior with the bundled code, and eliminates the source of these unhandled rejection events.

Legal Boilerplate

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Fixes JAVASCRIPT-2XH0

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