refactor(tests): Reduce code duplication in LLM tests

[iw4p]

Add helper function _call_complete_prompt() to eliminate repeated complete_prompt() calls with identical parameters across all test functions. This reduces test code by ~25% while maintaining the same test coverage.

Changes:

• Add _call_complete_prompt(temperature=0.0) helper function
• Replace 5 duplicate complete_prompt() calls with helper
• Reduce file from 104 to 79 lines (24% reduction)

This refactoring improves code maintainability by centralizing the common test parameters in a single helper function. Previously, each test function contained identical complete_prompt() calls with the same parameters, leading to code duplication and making future parameter changes error-prone. The helper function eliminates this duplication while preserving all existing test behavior and coverage.

Benefits:
Reduces code duplication by 24% (104 → 79 lines)
Improves maintainability - parameter changes only need to be made once
Maintains identical test behavior and coverage
Follows DRY (Don't Repeat Yourself) principle
No functional changes to test logic

Files changed:

• tests/sentry/llm/test_preview.py - Added helper function and refactored all test functions

Testing:

• All existing tests pass with no modifications to test logic

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