docs(escrow): document undocumented public methods in escrow services

[chainsmt]

Adds JSDoc to the 14 previously undocumented public methods across the escrow services so their behaviour and non-obvious constraints are captured at the source.

• buyer-dispute.service.ts: openDispute, getDispute
• escrow.service.ts: getTracking, createEscrow, createIdempotent, findById, getEvents, getPublicEscrow, findVendorEscrows, generateEvidenceUploadUrl, cancelEscrow, cancelPendingEscrow, handleShipment, syncStateFromChain

Each block explains what the method does and records what the signature alone does not convey: participant/state preconditions and the errors thrown, write/notification ordering, idempotency and caching behaviour, on-chain reconciliation rules, and deliberate non-actions (e.g. createEscrow does not fund, cancelEscrow never touches the chain, getEvents/findVendorEscrows perform no existence check or authorisation).

Documentation only — no behaviour, signature, or test changes.

Verification:

• npm run lint — passes (0 errors)
• npx tsc --noEmit — passes

Closes #661

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