feat(forex): add ForexContext for the currency-exchange channel (all layers)

[hogan-yuan]

Summary

New ForexContext for the currency-exchange (forex) channel — 3 endpoints
(quote / submit / query) — across all six SDK layers: Rust core (async +
blocking), C, C++, Java, Node.js, Python. Mirrors RFC-2026-0012 Appendix A.

Flow is three-step and asynchronous:

1. quote (POST /v1/forex/quote) → rate + quote_id
2. submit_order (POST /v1/forex/order) → accepted only (returns nothing)
3. order (GET /v1/forex/order) → poll by client_order_id to a terminal state

Design notes

• ForexOrderStatus is an integer-coded enum (Processing=0 / Success=1 /
  Failed=2), following the repo's existing num_enum + manual Serialize/Deserialize
  pattern; unknown values default to Processing.
• Money fields use Decimal (matching the trade channel). The query response's
  rate / from_amount / to_amount are nullable — they arrive as empty
  strings until the order is filled, decoded via the existing
  serde_utils::decimal_opt_empty_is_none helper.
• rate convention: standard currency pair (1 BASE = rate QUOTE), independent
  of the request's from/to direction; ccy_pair names the pair, expire_at is Unix ms.
• submit_order returns () — acceptance ≠ settlement; the idempotency contract
  (reuse same client_order_id on retry; each quote_id usable once → 603006) is
  documented on the method.
• No dedicated error-code enum (603001–603010 surface as the standard OpenApiError),
  consistent with the rest of the repo.
• C++ uses the real cbindgen header types directly (trade-style), no ffi mirror.
• Node.js/Python accept the quote amounts as decimal strings (fund-style), since
  async napi/PyO3 can't take a Decimal input directly; Python's from arg is from_.

Related (follow-ups, separate repos)

CLI / MCP / Docs are tracked separately, as with FundContext (#598).