test: err.rs code stability test

[Gbotemi-ojo]

Summary

Freezes all 93 client-facing error code discriminants in contracts/predictify-hybrid/src/err.rs with a dedicated stability test suite.

Changes

err_stability.rs (new test)

• 6 test functions covering every error variant: user operations (100-112), oracle (200-214), validation (300-304), general/state (400-441), override replay (526), and circuit breaker (500-527).
• total_variant_count() asserts exactly 93 variants exist, catching accidental insertions/deletions that would shift discriminants.
• All numeric values are frozen — any reordering, deletion, or insertion without an explicit discriminant will fail CI.

err.rs (bug fixes)

• Removed duplicate ReplayedOverride = 442 (conflicted with ReplayedOverride = 526).
• Removed dead match arms referencing non-existent ForceResolveReplayed and ForceResolveReasonEmpty.
• Fixed InsufficientStorageRent → OperationWouldExceedBudget in description().
• Added missing OperationWouldExceedBudget and ForceResolveAlreadyUsed to both description() and code().

Related fixes

• storage.rs, market_creation_validation_tests.rs: InsufficientStorageRent → InsufficientStorageRentBudget.

Verification

• rustfmt clean on all touched files.
• cargo test cannot run due to 245 pre-existing compilation errors in other modules (unrelated to this change).