Fix seer api auth header for delete by hash

[jennmueng]

Fixes "No auth header found" errors for Seer grouping record deletion by ensuring all Seer API calls are properly authenticated.

Why these changes?

Previously, call_seer_to_delete_these_hashes and call_seer_to_delete_project_grouping_records were making direct urlopen calls to Seer without the required authentication headers, leading to 401 errors.

What was changed?

• call_seer_to_delete_these_hashes: Switched from direct urlopen to make_signed_seer_api_request for authenticated POST requests.
• call_seer_to_delete_project_grouping_records: Converted the unauthenticated GET request to an authenticated POST request, aligning with other Seer API patterns and addressing a previous TODO.
• Tests: Updated existing mocks to target make_signed_seer_api_request and added new comprehensive tests for call_seer_to_delete_project_grouping_records.

This ensures all Seer deletion requests are signed and properly authenticated.

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.

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[cursor]

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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