feat(apigw): add non-orgid/slug endpoints to proxied cell requests

[gi0baro]

SSIA

[sentry]

Bug: The function get_cell_from_dsn hardcodes the number of domain segments, breaking error page embeds for deployments with different length domains (e.g., sentry.company.com or localhost).
_{Severity: HIGH}

Suggested Fix

The logic for determining the cell name from the DSN host should be dynamic. Instead of hardcoding the segment offset, calculate it based on the length of the system.url-prefix segments, similar to the previous implementation in src/sentry/hybridcloud/apigateway_async/proxy.py.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: src/apigw/dsl.py#L34-L41

Potential issue: The function `get_cell_from_dsn` incorrectly assumes the application's
host domain always consists of two segments (e.g., `sentry.io`). This is due to the
hardcoded logic `if (len(host_segments) - 2) < 3:` and `cell_segment =
host_segments[-3]`. For deployments with different domain lengths, such as self-hosted
instances on `sentry.company.com` or local development on `localhost`, this logic fails
to extract the correct cell name from the DSN. This causes `get_cell_by_name` to fail,
resulting in a 404 error and breaking the user feedback embed functionality for these
customers.

_{Did we get this right? 👍 / 👎 to inform future reviews.}