Phase 1: Fix 8 bugs, add Cancel Orders and Get Order endpoints

[milesangelo]

Summary

• Fix 8 confirmed bugs: CancellationToken propagation, OrderSearchRequest serialization, duplicate model types with type mismatches, non-cryptographic JWT nonce RNG, non-compiling examples, incorrect README, wrong SandboxBaseUrl default, untyped GetProduct return
• Add Cancel Orders endpoint (POST /orders/batch_cancel) with CancelOrdersAsync, CancelOrdersRequest/CancelOrdersResponse models
• Add Get Order by ID endpoint (GET /orders/historical/{orderId}) with GetOrderAsync, GetOrderResponse model
• Add typed GetProduct wrapper — GetProductAsync now returns AdvancedTradeProduct instead of raw string

Bug Details

#	Bug	Severity
B1	CancellationToken accepted but never passed to Polly or ICoinbaseApi	High
B7	OrderSearchRequest sent PascalCase query params instead of snake_case	High
B2	Duplicate order config types with decimal/double/string mismatches	Medium
B5	JWT nonce used new Random() instead of RandomNumberGenerator	Medium
B3	Examples called non-existent methods and used wrong property names	Medium
B4	README showed non-existent CoinbaseApiException, wrong OrderSide.Buy	Medium
B6	SandboxBaseUrl defaulted to localhost instead of Coinbase sandbox	Low
B8	GetProduct returned Task<string> instead of typed model	Low

Test plan

• 12 new unit tests added (CancelOrders, GetOrder, GetProduct, CancellationToken propagation, serialization)
• All 62 tests passing (48 unit + 14 integration)
• dotnet build — 0 errors
• dotnet format --verify-no-changes — clean
• No breaking changes to existing public API (CancellationToken params have defaults)

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