Add JSDoc to all `YieldService` public methods

[Jayy4rl]

Context: Same requirement as Issue 253 for YieldService.

Requirements:

• Add JSDoc to getVaultEpochs, getUserPendingYield, recordEpoch, getVaultYieldSummary.
• Document the BigInt arithmetic in getUserPendingYield with a @remarks tag.

Acceptance criteria:

• TypeScript language server shows JSDoc tooltips for all four methods.