fix: add Gtd variant to TimeInForce enum

[hotea]

Summary

• Add Gtd variant (serialized as "GTD") to TimeInForce enum in src/model/enums.rs
• Add serialize/deserialize tests for the new variant

Root Cause

TimeInForce only had Day, Gtc, Opg. Gtd (Good-Till-Date) was documented but missing from the enum, making it impossible to construct GTD orders with the typed enum.

Test plan

• cargo test passes with new GTD serialize/deserialize assertions
• Place a GTD order using TimeInForce::Gtd and verify it serializes to "GTD"