fix(cursor): Handle empty branch_name when launching Cursor agent

[JoshFerge]

Summary

When branch_name is an empty string, the Cursor API returns an error:

"Git ref (branch/tag) cannot be an empty string"

This fix converts empty/falsy branch_name to None, which gets excluded from the JSON payload via exclude_none=True, allowing Cursor to automatically resolve to the repository's default branch.

Root Cause Analysis

The issue occurs when:

1. A repo is configured in Seer preferences (with branch_name left blank, expecting default branch to be used)
2. The Sentry GitHub app is NOT installed on the repo (or lacks read permissions)
3. Seer's update_repo_access_and_properties() cannot resolve the default branch because the repo is not readable
4. branch_name stays empty
5. Cursor API receives "ref": "" and rejects it

Note: When the Sentry GitHub app HAS read access, Seer correctly resolves branch_name to the default branch during autofix run creation.

Why This Fix Helps

This is a workaround for cases where:

• User has the Cursor GitHub app installed (so Cursor can access the repo)
• But forgot to install the Sentry GitHub app (so Seer can't resolve the default branch)

Scenario	Without fix	With fix
Sentry app has access	✅ Works	✅ Works
Sentry app NO access, Cursor HAS access	❌ Fails (ref: "")	✅ Works (Cursor resolves default)
Neither app has access	❌ Fails	❌ Still fails

The proper fix is ensuring users install the Sentry GitHub app on their repos. A companion PR adds logging in Seer to help identify when this happens: https://github.com/getsentry/seer/pull/4619

Verified with Cursor API

# Empty string → Error
curl ... --data '{"source": {"repository": "...", "ref": ""}}'
# {"error": "Invalid creation request parameters", "details": [{"message": "Git ref (branch/tag) cannot be an empty string"}]}

# Omitted → Success (resolves to default branch)
curl ... --data '{"source": {"repository": "..."}}'
# {"source": {"ref": "main"}, ...}  ✓

Test plan

• Added tests for empty string and None branch_name cases
• Verified existing tests still pass
• Manually tested against Cursor API

[linear]

AIML-2353 Seer fails when Cursor agent receives empty source.ref

[JoshFerge]

@sentry review
bugbot review

[JoshFerge]

bugbot review

[JoshFerge]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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