fix(api): handle rejected fetch callbacks

[sansynx]

Fixes #126120

Current master already rejects requestPromise() through its fallback, but a rejected fetch still skips direct request() error and completion callbacks.

Route non-abort fetch failures through the existing error and completion handlers with status 0 and no JSON body, then clean up the active request. Keep these failures classified as RequestError so the SDK's no-response filter continues to apply to main errors and causes. Intentional cancellations remain silent, and server errors remain reportable.

Validation:

• API, SDK initialization/filtering, and unchanged replay suites: 177 tests passed
• Before the follow-up fix, six regression cases failed for incorrect error classification and filtering across GET, POST, PUT, PATCH and DELETE
• Oxfmt checks passed for all affected files
• Affected-file lint passed with SENTRY_OXLINT_TYPEAWARE=false and --threads 1
• Independent diff review and git diff --check passed

Tests used the existing Jest runner, standard setup and aliases, pinned Linux Node 24.14.0 and a private cache. Full local typechecking, application build and browser verification were not run. The earlier local prek attempt stalled before its hooks executed; CI pre-commit lint, TypeScript and oxlint passed on the previous commit. The new push will rerun CI.

AI assistance was used to prepare and validate this change

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[sansynx]

could a collaborator add the Trigger: getsentry tests label after reviewing this and authorize the Vercel preview? those checks are waiting on approval