gh-59598: Ignore leading whitespace in JSONDecoder.raw_decode

[mayank-dev-15]

Summary

Fixes #59598

JSONDecoder.raw_decode() raises JSONDecodeError on strings with leading whitespace (e.g., ' {"key": "value"}'), while JSONDecoder.decode() handles it fine. This is inconsistent behavior.

Changes

• Modify raw_decode() to accept an optional _w parameter (defaulting to WHITESPACE.match) to skip leading whitespace before decoding
• Simplify decode() to call raw_decode(s) instead of manually computing the whitespace end index
• The fix correctly handles the idx parameter — whitespace is skipped at any offset, not just the start

Before

>>> decoder = json.JSONDecoder()
>>> decoder.raw_decode('  {}')
JSONDecodeError: Expecting value: line 1 column 1 (char 0)

After

>>> decoder = json.JSONDecoder()
>>> decoder.raw_decode('  {}')
({}, 3)

Tests

Added TestRawDecode test class with tests for leading whitespace in raw_decode, covering both Python and C implementations.

• Issue: JSONDecoder.raw_decode breaks on leading whitespace #59598

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