fix: coinjoin wallet manager lock order

[PastaPastaPasta]

Issue being fixed or feature implemented

Fixes a debug lock-order assertion seen in CI when CoinJoin wallet-manager maintenance and wallet keypool/client lookup paths observe opposite lock orders.

The problematic cycle was:

• wallet/keypool path: CWallet::cs_wallet -> cs_wallet_manager_map
• CoinJoin maintenance/teardown path: cs_wallet_manager_map -> CoinJoin manager/session code -> CWallet::cs_wallet

This can abort debug builds with a potential deadlock / inconsistent lock order assertion.

What was done?

• Store CoinJoin client managers with std::shared_ptr ownership instead of std::unique_ptr.
• Limit cs_wallet_manager_map to short-lived map/index operations: snapshot, lookup, insert, extract/erase, and destructor swap.
• Run manager callbacks, maintenance, reset/stop, and teardown after releasing cs_wallet_manager_map.
• Make CoinJoinClientImpl hold a shared_ptr so returned interface clients cannot dangle after wallet removal.
• Add regression coverage for the newkeypool callback path that stops CoinJoin mixing while holding the wallet lock.

How Has This Been Tested?

Ran the checks available in this checkout:

• git diff --check upstream/develop..HEAD
• PYTHONPYCACHEPREFIX=/private/tmp/dash-pycache python3 -m py_compile test/functional/rpc_coinjoin.py
• Static audit for LOCK(cs_wallet_manager_map) and WITH_LOCK(cs_wallet_manager_map) to confirm remaining map locks only protect map swap/snapshot/lookup/insert/extract.

Full unit/functional execution was not run locally because this checkout has no configured build tree or binaries (Makefile, src/test/test_dash, and src/dashd are absent).

Breaking Changes

None. This changes internal CoinJoin wallet-manager ownership/lifetime semantics only; there are no RPC, wallet-file, network, or user-facing API changes.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation
• I have assigned this pull request to a milestone (for repository code-owners and collaborators only)

[github-actions]

✅ No Merge Conflicts Detected

This PR currently has no conflicts with other open PRs.

[knst]

I refactored this part significantly: https://github.com/dashpay/dash/pull/7259/files
Let me revive my PR instead.