Collapse the triplicated DisputeResolution, raise_dispute, and resolve_dispute definitions

[Baskarayelu]

Closes #655

Summary

• Remove duplicate mod amount_validation declaration and duplicate safe_add_amounts function from lib.rs
• Remove duplicate DisputeResolution re-export (was exported from both dispute and types; now only from types)
• Remove duplicate EscrowError contracterror enum in lib.rs; replaced by pub type EscrowError = Error aliasing the canonical Error from types.rs
• Remove duplicate #[contractimpl] impl Escrow block from dispute.rs: raise_dispute and resolve_dispute are now defined exactly once, in lib.rs
• Remove duplicate DataKey enum from types.rs (second definition was byte-for-byte identical)
• Remove duplicate Error enum from types.rs; merge missing EscrowCapExceeded into canonical first definition with a non-colliding discriminant (51 instead of 50 which is taken by AlreadyCancelled)
• Add DisputeResolution::code() method on the canonical type in types.rs (used by the resolve_dispute event payload)
• Rewrite test/dispute.rs with focused, self-contained tests covering all four resolution variants (FullRefund, FullPayout, PartialRefund, Split), security gates (non-party caller, no arbiter, non-disputed state, double resolve), and the conservation invariant

Security notes

All security invariants are preserved on the single code path:

• Pause/emergency gate fires before any state mutation
• Only client or freelancer may raise a dispute
• Arbiter must be assigned before a dispute can be raised
• Only the assigned arbiter may resolve a dispute
• Resolution only allowed on Disputed contracts
• Conservation invariant released_amount + refunded_amount == funded_amount is asserted after every resolution

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[mikewheeleer]

appreciate this — thanks for picking it up 🙌