Fix governance policy wire canonicality

[carrion256]

Summary

• Reject SetGovernancePolicy payloads that carry fields irrelevant to the selected policy kind.
• Adds regression coverage proving sparse governance policy actions no longer have multiple accepted wire encodings for the same effective action.
• Updates an existing integration test to use a registered governance contract, matching the branch's production topology validation.

Findings

• #FIND-092 / Nexus 28360a4f-b33c-45fd-bef9-67a7acbf0dc0
• #FIND-090 / Nexus 109dc6ff-46cf-4974-bec7-089886e8560c is triaged as not requiring a code change: shared-types already exposes distinct CodecError variants and enforces strict cursor exhaustion; the public contract boundary intentionally maps malformed client payloads to compact InvalidInput.

Verification

• cargo test -p templar-soroban-runtime runtime_governance_policy_rejects_irrelevant_fields --test integration_tests -- --nocapture
• cargo test -p templar-soroban-runtime --test integration_tests -- --nocapture
• post-commit Soroban size-budget-check hook passed: 97270 bytes

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