fix: require governance group market id

[carrion256]

Fixed Findings

• A-062 / Nexus e019c107-ebbe-4cbd-8446-06739d060ac0

Summary

• Require market_id for SetGovernancePolicy group-membership commands (mode == 2).
• Preserve existing group absolute-cap and relative-cap behavior, which does not use market_id.
• Add a regression proving omitted market_id now returns ContractError::InvalidInput and does not silently update market 0.

RED Evidence

Before the fix, the focused regression failed because a group-membership payload with market_id: None returned Ok(()):

cargo test -p templar-soroban-runtime test_execute_governance_group_membership_requires_market_id -- --nocapture
left: Ok(())
right: Err(InvalidInput)

Verification

• cargo test -p templar-soroban-runtime test_execute_governance_group_membership_requires_market_id -- --nocapture
• cargo test -p templar-soroban-runtime --lib -- --nocapture (104 passed)
• cargo fmt --check
• git diff --check
• cargo test -p templar-soroban-runtime -- --nocapture
• just -f contract/vault/soroban/justfile build
• just -f contract/vault/soroban/justfile size-budget-check (93946 <= 131072 bytes)

Stack / Base

• Base: PR 417 branch spr/refactor/vault-ergonomics/4f330057
• Branch: audit/governance-abi-validation-a062

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