feat: add structured json output support for kcl vet command

[Arpit529Srivastava]

1. Does this PR affect any open issues?(Y/N) and add issue references (e.g. "fix #123", "re #123".):

• N
• Y

fix: kcl-lang/kcl#2043

2. What is the scope of this PR (e.g. component or file name):

cmd/kcl/commands/vet.go

3. Provide a description of the PR(e.g. more details, effects, motivations or doc link):

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Other

This PR adds a --output flag to the kcl vet command, enabling structured output for validation results.

• Supports json output format.
• Default behavior (text output) remains unchanged.
• Parses the underlying KCL runtime error string into a structured JSON object containing file, line, column, message, and errorType.
• Strips ANSI color codes from the JSON output for cleaner processing.

Motivation:
Currently, kcl vet only outputs unstructured human-readable text which is difficult to parse in CI/CD pipelines. This change allows tools to programmatically consume validation results.

Example Usage:

kcl vet data.json code.k --output json

Output:

{
  "success": false,
  "errCount": 1,
  "errors": [
    {
      "errorType": "EvaluationError",
      "file": "path/to/file.yaml",
      "line": 3,
      "column": 9,
      "message": "Instance check failed",
      "codeSnippet": "app_name: \"test\""
    }
  ]
}

4. Are there any breaking changes?(Y/N) and describe the breaking changes(e.g. more details, motivations or doc link):

• N
• Y

5. Are there test cases for these changes?(Y/N) select and add more details, references or doc links:

• Unit test
• Integration test
• Benchmark (add benchmark stats below)
• Manual test (add detailed scripts or steps below)
• Other

Verified manually with both passing and failing validation scenarios:

1. Created a schema file and data file.
2. Ran kcl vet data.yaml schema.k --output json.
3. Verified the JSON structure for errors (including single and multiple errors).
4. Verified that ANSI codes are stripped from the messages.

[coveralls]

Pull Request Test Coverage Report for Build 20861931473

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 27.386%

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Totals
Change from base Build 20693487568:	0.0%
Covered Lines:	241
Relevant Lines:	880

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💛 - Coveralls

[Arpit529Srivastava]

@Peefy PTAL.
Thanks