test: cross-variant DataKey collision checker

[Clinton6801]

Summary

Adds a generative collision-detection test for the DataKey enum
in storage.rs.

Changes

• contracts/predictify-hybrid/tests/datakey_collision.rs (new)
  Constructs all 3 DataKey variants, encodes each via env.to_xdr(),
  and compares pairwise — fails with a clear message naming any
  colliding pair.

• contracts/predictify-hybrid/src/storage.rs
  Added doc comment to DataKey enum referencing the collision test.

• contracts/predictify-hybrid/Cargo.toml
  Registered the new integration test target.

Test output

[paste the terminal output Kiro showed you here]

Acceptance criteria

✅ Test fails if two variants encode identically
✅ Clear failure output naming the colliding pair
✅ No unwrap() — uses expect() throughout
✅ Documented in storage.rs
✅ No format!() for building keys
✅ Only 3 files changed

closes #624

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