Fix probabilistic auth header omission for seer api

[jennmueng]

Fixes intermittent "No auth header found" errors for Seer API requests.

The root cause was a probabilistic check in `sign_with_seer_secret` that could omit the `Authorization` header, even though Seer requires it for all requests. This change removes that probabilistic logic, ensuring authentication headers are always included when `settings.SEER_API_SHARED_SECRET` is configured.

This guarantees reliable authentication and eliminates intermittent failures. Tests have been updated to reflect this consistent behavior, including a new test to verify headers are always added.

### Legal Boilerplate

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. and is gonna need some rights from me in order to utilize my contributions in this here PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.
</pr_request_template>

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Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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