test: add spec test for duplicate variant materialization assignments

[nicklasl]

Summary

• Adds a spec test case where two assignments in a materialization rule point to the same variant but have different bucket ranges and assignment IDs
• Verifies the resolver correctly picks the right assignment based on bucket matching

Test plan

• cargo test -p confidence_resolver mat_same_variant passes

🤖 Generated with Claude Code

[yyoyoian-pixel]

LGTM — targets a real and subtle edge case (two assignments for the same variant in different bucket ranges). The new assignment_id assertion infrastructure via resolve token decryption is a useful addition to the spec test framework.

Optional suggestion: a complementary test with a different targeting_key that hashes into the 0-500 range (expecting a1) would fully exercise both assignments and guard against boundary bugs.

[vahidlazio]

LGTM — targets a real and subtle edge case (two assignments for the same variant in different bucket ranges). The new assignment_id assertion infrastructure via resolve token decryption is a useful addition to the spec test framework.

Optional suggestion: a complementary test with a different targeting_key that hashes into the 0-500 range (expecting a1) would fully exercise both assignments and guard against boundary bugs.