Improve error messages when database access is disposed (#174)

[konard]

Summary

This PR fixes GitHub issue #174 by implementing better error messages when database access is disposed right after the access is created.

Problem

Previously, when a Links instance was disposed and users tried to access database methods (Count(), Create(), Update(), Delete(), Each()), they would encounter:

• Generic null reference exceptions
• Access violations
• Non-descriptive error messages
• Crashes with unclear stack traces

This made debugging difficult for users who accidentally tried to use disposed database connections.

Solution

Added comprehensive disposal checking to all public database access methods across all Links implementations:

Key Changes:

• Added EnsureNotDisposed() method to all Links base classes:

  • UnitedMemoryLinksBase<T>
  • SplitMemoryLinksBase<T>
  • Links<T> (FFI)
  • UInt32Links (FFI)

• Clear, descriptive error message:

  "Cannot access a disposed Links instance. The database connection has been closed and is no longer available for operations."
  

• Applied to all public methods:

  • Count(IList<TLinkAddress>? restriction)
  • Create(IList<TLinkAddress>? substitution, WriteHandler<TLinkAddress>? handler)
  • Update(IList<TLinkAddress>? restriction, IList<TLinkAddress>? substitution, WriteHandler<TLinkAddress>? handler)
  • Delete(IList<TLinkAddress>? restriction, WriteHandler<TLinkAddress>? handler)
  • Each(IList<TLinkAddress>? restriction, ReadHandler<TLinkAddress>? handler)

Testing

• Added comprehensive test suite (DisposalErrorMessagesTests.cs) that verifies:
  • All methods throw ObjectDisposedException when called on disposed instances
  • Exception messages contain meaningful descriptions
  • Object names are correctly included in exceptions
  • Non-disposed instances continue to work normally
• All existing tests pass - no regressions introduced

Example Usage

Before (unclear error):

var links = new UnitedMemoryLinks<uint>(memory);
links.Dispose();
var count = links.Count(); // NullReferenceException or similar

After (clear error):

var links = new UnitedMemoryLinks<uint>(memory);
links.Dispose();
var count = links.Count(); 
// ObjectDisposedException: Cannot access a disposed Links instance. 
// The database connection has been closed and is no longer available for operations.

Impact

• Better Developer Experience: Users get immediate, clear feedback when they accidentally use disposed connections
• Easier Debugging: Error messages explain exactly what's wrong and what needs to be fixed
• Consistent Behavior: All Links implementations now behave the same way when disposed
• Zero Breaking Changes: Only adds new error checking, doesn't change existing functionality

Test plan

• All existing unit tests pass
• New disposal error tests pass
• Manual testing with disposed instances shows improved error messages
• Build succeeds with no warnings or errors

🤖 Generated with Claude Code

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Resolves #174