[P1] Insufficient vault validation - only checks existence, not ownership or status

Resolve #2447

This MR implements the changes described in issue #2447.

Summary

• Add vault type (Asgard) and status validation in ensureVaultAndGetTxInVoter function
• Reject NEW transactions to inactive vaults early, before voter creation and balance updates
• Allow existing observations to continue processing (refund handled later in flow)
• Add comprehensive unit tests for all vault validation scenarios

Technical Details

The security finding identified that ensureVaultAndGetTxInVoter only validated vault existence, not:

1. Vault type (should be Asgard)
2. Vault status (Active/Retiring/Inactive)

This allowed transactions to inactive vaults to proceed through voter creation and vault balance updates (lines 189-193) before being rejected at line 263. The fix adds early validation to:

1. Retrieve vault early and check it's an Asgard vault (reject non-Asgard)
2. Check vault status for NEW observations (no existing voter) - reject inactive vaults immediately
3. Allow existing observations to continue (vault may have become inactive after initial observation)

The distinction between new and existing observations is important:

• New observations: Should be rejected immediately for inactive vaults (security fix)
• Existing observations: Should continue to process (vault was active when first observed, refund handled in handleObservedTxInQuorum)

Test Plan

• Unit tests for active Asgard vault (should succeed)
• Unit tests for inactive vault with no prior observations (should reject early)
• Unit tests for inactive vault with existing observations (should allow processing)
• Unit tests for non-Asgard vault (should reject)
• Unit tests for retiring vault (should succeed)
• Unit tests for non-existent vault (should reject)
• Run make test to verify no regressions
• Run make lint to verify code quality

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