Fix CLI error detail and BOM handling for JSON input

[vinitkumar]

Summary

Three fixes, one commit each, plus a small test follow-up.

• Surface the decoder's line and column in reader errors so the CLI says where the input breaks. Fixes CLI hides the JSON parser's line and column on invalid input #436
• Accept a UTF-8 byte order mark in file, URL, and string input. Fixes JSON input with a UTF-8 BOM is rejected as invalid #437
• Fix the 16 diagnostics ty 0.0.83 reports in tests so the typecheck job passes again. Fixes Typecheck fails with ty 0.0.83: 16 diagnostics in tests #439

Before:

$ json2xml-py examples/wrongjson.json
Error: Could not parse JSON file: examples/wrongjson.json. Check that the file contains valid JSON. (Invalid JSON File)

After:

Error: Could not parse JSON file: examples/wrongjson.json. Check that the file contains valid JSON. (Invalid JSON File: Expecting ',' delimiter: line 372 column 3 (char 8920))

Error prefixes are unchanged, so existing callers matching on them are unaffected. No library code changed for #439; the CustomNumber test helpers gain __hash__, the affected payloads are declared as JSONValue, and one test that asserted a bare literal now compares the result.

Test plan

• Regression tests written first and observed failing, then passing (8 new tests: file, string, URL, and end-to-end CLI)
• make test on CPython: 100% coverage; full suite also passes on PyPy 3.10
• make lint and make typecheck clean with ty 0.0.83
• lat check passes; lat.md/tests.md and lat.md/behavior.md updated

Summary by Sourcery

Improve JSON input diagnostics and compatibility by preserving decoder locations and accepting UTF-8 BOMs across all readers.

New Features:

• Accept leading UTF-8 byte order marks in file, URL, and string JSON input.

Bug Fixes:

• Include JSON decoder error details, including line and column, in reader errors so the CLI identifies where malformed input fails.
• Update affected test fixtures and helpers for compatibility with ty 0.0.83.

Documentation:

• Document decoder positions in invalid-input errors and UTF-8 BOM support for JSON readers.

Tests:

• Add regression coverage for decoder positions, BOM-prefixed file, URL, string, and CLI input.

[sourcery-ai]

Reviewer's Guide

Updates all JSON readers to accept leading UTF-8 BOMs and preserve decoder error details, including line and column positions, while maintaining existing error prefixes; adds end-to-end coverage and behavioral documentation.

Flow diagram for BOM-tolerant JSON input and detailed errors

flowchart LR
    File["File input"] --> FileReader[readfromjson]
    URL["URL input"] --> URLReader[readfromurl]
    String["String or stdin input"] --> StringReader[readfromstring]
    FileReader --> Decode["Decode JSON after removing leading BOM"]
    URLReader --> Decode
    StringReader --> Decode
    Decode --> Parsed["Parsed JSON"]
    Decode --> Error["Source-specific error with decoder line and column"]
    Error --> CLI["CLI displays unchanged error prefix plus details"]

Loading

File-Level Changes

Change	Details	Files
Preserve JSON decoder diagnostics in source-specific reader errors.	Append the underlying parse exception to file, URL, and string reader messages. Retain existing error prefixes for caller compatibility. Verify line/column details through reader and CLI regression tests.	json2xml/utils.py tests/test_utils.py tests/test_cli.py
Support leading UTF-8 BOMs across all JSON input paths.	Use utf-8-sig for file decoding. Decode URL response bytes with utf-8-sig. Strip a leading U+FEFF from string input before parsing. Cover BOM handling for files, URLs, strings, and the CLI.	json2xml/utils.py tests/test_utils.py tests/test_cli.py
Document the updated reader behavior and test expectations.	Document decoder positions in wrapped errors. Document BOM acceptance for all three readers. Link regression tests to the corresponding behavioral requirements.	lat.md/behavior.md lat.md/tests.md

Assessment against linked issues

Issue	Objective	Addressed	Explanation
#436	Preserve and expose the JSON decoder's detailed error message, including line and column information, when malformed file, string, URL, or stdin JSON is wrapped by the input readers and reported by the CLI.	✅
#437	Accept JSON input prefixed with a UTF-8 BOM when reading from files.	✅
#437	Accept JSON input containing a leading U+FEFF when reading from strings, including stdin and --string input.	✅
#437	Accept JSON input prefixed with a UTF-8 BOM when reading response bodies from URLs.	✅

Possibly linked issues

• CLI hides the JSON parser's line and column on invalid input #436: The PR directly appends decoder messages to file, string, and URL errors, enabling CLI line and column reporting.
• JSON input with a UTF-8 BOM is rejected as invalid #437: PR changes file and URL decoding to utf-8-sig and removes leading U+FEFF from strings, exactly fixing issue JSON input with a UTF-8 BOM is rejected as invalid #437.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a5d436d) to head (0fe2f5a).
⚠️ Report is 13 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #438   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines         1032      1032           
=========================================
  Hits          1032      1032           

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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