fix(llm-detector): Use project's platform instead of "other"

[roggenkemper]

uses the project's platform instead of other for llm detected issues. also removes unnecessary tag

fixes https://linear.app/getsentry/issue/ID-1080/add-platform-to-occurrences

[linear]

ID-1080 Add platform to occurrences

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
29556	1	29555	242

View the top 1 failed test(s) by shortest run time

tests.sentry.tasks.test_llm_issue_detection.LLMIssueDetectionTest::test_create_issue_occurrence_from_detection

Stack Traces | 2.44s run time

#x1B[1m#x1B[.../sentry/tasks/test_llm_issue_detection.py#x1B[0m:123: in test_create_issue_occurrence_from_detection
    assert event_data["tags"]["llm_detected"] == "true"
#x1B[1m#x1B[31mE   KeyError: 'llm_detected'#x1B[0m

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