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

[JoshFerge]

Summary

Fixes AIML-2353

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

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

This affects ~50% of Seer project preferences where users intentionally leave branch_name blank, expecting Seer to use the repo's default branch (as per the UI copy: "if left blank Seer will look at the default branch of the repository").

Root Cause

• Repos stored in Seer preferences can have branch_name="" (empty string)
• The Cursor client passed branch_name directly to ref
• Empty strings weren't excluded by exclude_none=True (only None is excluded)
• Cursor API received "ref": "" and failed validation

Fix

Convert empty/falsy branch_name to None:

ref=request.repository.branch_name or None

This causes the field to be excluded from the JSON payload, allowing Cursor to automatically resolve to the repository's default branch.

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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