feat: add custom results methods to Project

[tchordia]

Summary

(to be filled in)

Claude Notes

Adds four Project classmethods to manage a project's custom HTML results file via the new api_external custom-results endpoints (backend PR: surge-ai/gondor#36742). Uses a presigned-URL upload (no multipart).

• Project.upload_custom_results(project_id, file) — POSTs for a presigned PUT URL, then PUTs the file (path or file-like) to S3 with Content-Type: text/html. Re-upload overwrites (fixed key).
• Project.get_custom_results_url(project_id) — returns a presigned download URL.
• Project.download_custom_results(project_id) — downloads and returns the HTML content.
• Project.delete_custom_results(project_id) — deletes the file.

The two read methods are intentionally separate (URL vs. content). All read methods sit on the single GET endpoint, which 404s when absent (raising SurgeRequestError).

Test plan

• pytest tests/test_projects.py — 5 new tests + all pre-existing pass (15 total)
• Covers upload (path + file-like; asserts Content-Type: text/html), get-url, download (utf-8 decode), delete
• No new ruff findings introduced (surge/projects.py is clean)
• Reviewer sanity check

Note: requires the backend PR (gondor#36742) to be deployed before these endpoints exist.

[tchordia]

• feat: add custom results methods to Project #186 👈 (View in Graphite)
• main

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