Throw error when PostgresCopyFromWriter is escaped

[ahoppen]

When PostgresCopyFromWriter is escaped from the copyFrom closure (which users really shouldn’t do) and the user tries to write data to it after copyFrom has finished, throw an error instead of hitting a preconditionFailure.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 75.59%. Comparing base (4db2fde) to head (13209ec).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...nection State Machine/ConnectionStateMachine.swift	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #605      +/-   ##
==========================================
+ Coverage   75.42%   75.59%   +0.16%     
==========================================
  Files         134      134              
  Lines        9887     9914      +27     
==========================================
+ Hits         7457     7494      +37     
+ Misses       2430     2420      -10     

Files with missing lines	Coverage Δ
...esNIO/Connection/PostgresConnection+CopyFrom.swift	93.22% <100.00%> (+0.11%)	⬆️
...tion State Machine/ExtendedQueryStateMachine.swift	78.69% <100.00%> (+0.21%)	⬆️
...nection State Machine/ConnectionStateMachine.swift	68.75% <0.00%> (ø)

... and 3 files with indirect coverage changes

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