Add regression test FIX

[Mitch5000]

TITLE

test: add regression test for deregister/re-register preserving balances and provider counts

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DESCRIPTION

Summary

Adds a regression test that explicitly validates the documented behavior of deregister_anchor:

"Existing pool liquidity is unaffected; the anchor simply cannot open new positions."

The test confirms that a deregister → re-register cycle for an anchor that has previously provided liquidity:

• Leaves balance(), anchor_balances(), and pool.providers completely unchanged
• Does not reset or double-count providers
• Allows the anchor to immediately withdraw_liquidity (or withdraw_all_liquidity) using the pre-existing balance
• Correctly blocks provide_liquidity and open_settlement with AnchorNotRegistered while the anchor is deregistered

No production code changes were required — this closes a test-coverage gap.

Changes

• Added test_deregister_re_register_preserves_balances_and_provider_count in src/test.rs
• Test is placed directly after the existing test_list_anchors_reflects_reregistration test (as suggested in the issue)
• Reuses existing helpers (setup, register_anchor, deregister_anchor, etc.)
• Covers all acceptance criteria from the issue description

Test Coverage Added

The new test asserts:

1. Initial state after provide_liquidity
2. State after deregister_anchor (balances + provider count preserved)
3. provide_liquidity and open_settlement are rejected with AnchorNotRegistered while deregistered
4. State after re-register_anchor is identical to before deregistration (no reset, no double-count)
5. withdraw_all_liquidity succeeds immediately after re-registration using the preserved balance

Motivation / Security Note

As stated in the issue:

If deregistration ever accidentally touched balance state in a future refactor, an anchor's funds could become temporarily or permanently inaccessible purely from an administrative registration action — a serious fund-availability risk this test explicitly guards against.

How to Verify

cargo test test_deregister_re_register_preserves_balances_and_provider_count -- --nocapture
cargo test

Closes #156