feat(governance): dispatch factory rate and pause calldata

[vku2018]

Closes #818.

Summary

• add FactorySetRateBounds(Option, Option) and FactorySetPaused(bool) to governance CallData
• dispatch those variants to set_rate_bounds and set_factory_paused on the target factory contract
• add XDR round-trip coverage and mock-target proposal/approve/execute tests for both variants

Validation

g -n "FactorySetRateBounds|FactorySetPaused|set_rate_bounds|set_factory_paused|test_factory_policy_calldata_round_trips_xdr|test_factory_rate_bounds_calldata_dispatches|test_factory_paused_calldata_dispatches" contracts/governance/src/lib.rs

• git diff --check
• cargo test -p fluxora_governance is blocked locally because Cargo is not installed in this environment

ustfmt --version is blocked locally because rustfmt is not installed in this environment