Make NU6.3 wallet test setup exception-safe

[p0mvn]

Summary

• install a height-0 fake chain tip before calling PrepareTransaction
• select the first valid Sapling diversifier instead of assuming index 0 is valid
• use scoped cleanup for the fake chain tip, network upgrade, and global wallet state

Motivation / Why

The NU6.3 wallet-selection test originally ran with chainActive at height -1, causing consensus branch lookup to assert before the test reached its expectations.

The initial chain-tip fix exposed a second latent failure: Sapling diversifier index 0 is not valid for every randomly generated wallet seed. When .value() threw bad_optional_access, manual cleanup was skipped. That left a pointer to a stack-allocated CBlockIndex in global state, which caused the following test to observe height 0 and could make global teardown abort with free(): invalid pointer.

This PR makes all temporary test state exception-safe and uses FindAddress to obtain a valid Sapling address.

Tests fixed

• WalletRPCTests.PrepareTransactionAvoidsOrchardAfterNU6point3
  • assertion from passing chain height -1 into consensus upgrade lookup
  • intermittent bad_optional_access from an invalid Sapling diversifier
• WalletRPCTests.RPCZsendmanyTaddrToSapling
  • collateral failure from the preceding test leaking a height-0 chain tip
• GoogleTest process teardown
  • invalid free caused by a leaked pointer to a stack CBlockIndex

Validation

• local x86_64 macOS build via ./zcutil/build.sh
• affected test passed 100 consecutive randomized repetitions
• affected test and RPCZsendmanyTaddrToSapling passed sequentially
• all CI checks pass, including every Ubuntu, Debian, ARM64, and Windows GoogleTest shard