Fix parser error pointing to wrong line at EOF without trailing newline

[pelletier]

Summary

• Fix error highlight pointing to line 1 instead of the correct line when parsing a key missing = at EOF without a trailing newline (e.g., a = 1\nb = 2\nc)
• The root cause was NewParserError receiving an empty byte slice as the highlight, causing subsliceOffset to return 0. Now passes the consumed key bytes (startB) as the highlight instead.
• Add regression test for this scenario

Fixes #1032

Error output example

Given this input (no trailing newline after c):

a = 1
b = 2
c

Before (error incorrectly points to line 1):

1| a = 1
 |  expected = after a key, but the document ends there
2| b = 2
3| c

After (error correctly points to line 3):

1| a = 1
2| b = 2
3| c
 | ~ expected = after a key, but the document ends there

Test plan

• All existing tests pass (go test ./...)
• New test case TestDecodeError_Position/missing_equals_on_last_line_without_trailing_newline verifies error points to line 3
• Manual verification confirms correct error output as shown above

https://claude.ai/code/session_01UWv8pyc8P1ktAPfHpveixj