[WIP] Fix poor error message on cast effect violation

[Copilot]

Improve error message for cast effect violations

• Understand the issue and reproduce the error
• Locate error message generation in sempass2.nim
• Analyze how cast(raises: ...) blocks are tracked
• Improve error message to reference the cast block
• Test the fix
• Run code review and security checks

Original prompt

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This section details on the original issue you should resolve

<issue_title>Poor error message on cast effect violation</issue_title>
<issue_description>### Description

proc fff() {.raises: [].} =
  {.cast(raises: ValueError).}:
    echo "hello"

Nim Version

1.6.16, devel

referencing the cast would be nicer

Current Output

testit.nim(2, 18) Error: ValueError can raise an unlisted exception: ValueError

Expected Output

No response

Possible Solution

No response

Additional Information

No response</issue_description>

<agent_instructions>use ./koch temp c -r test.nim to debug this case, see whether you can improve the error messages as described</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Poor error message on cast effect violation #22950

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