mark expression scanner: backslash check in string literal searches entire input instead of the string value

Was testing `-k` with a test name that contains backslash-escaped characters (like the `\n` you get from parametrized test IDs) alongside a marker expression with string arguments, and ran into a false rejection. The scanner in `src/_pytest/mark/expression.py` line 105: ```python if (backslash_pos := input.find("\\")) != -1: raise SyntaxError( r'escaping with "\\" not supported in marker expression', (FILE_NAME, 1, backslash_pos + 1, input), ) ``` This searches the **entire input** for a backslash, but it should only search within the current string literal (`value`). The original code before commit 1e7eb2034 used `"\\" in value` which was correct. The refactoring to `input.find("\\")` accidentally changed the search scope. This means any expression that has a backslash in an identifier (which is valid per #8983) AND also contains a string literal will be incorrectly rejected: ``` $ pytest -k 'test\nfoo\n and mark(x="y")' ERROR: Wrong expression passed to '-k': test\nfoo\n and mark(x="y"): at column 5: escaping with "\" not supported in marker expression ``` The backslash at position 4 is in the identifier `test\nfoo\n`, not inside the string literal `"y"`. But `input.find("\\")` finds it anyway because it scans the whole input. Repro: ```python from _pytest.mark.expression import Expression # Works fine -- backslash in identifier, no string literal Expression.compile(r'test\nfoo\n') # ok # Works fine -- backslash inside string literal, correctly rejected try: Expression.compile(r"mark(var='\hello')") except SyntaxError: pass # expected # Broken -- backslash in identifier + string literal = false rejection Expression.compile(r'test\nfoo\n and mark(x="y")') # SyntaxError! ``` The fix is changing `input.find("\\")` to `value.find("\\")` and adjusting the column offset from `backslash_pos + 1` to `pos + backslash_pos + 1` so the error points to the right place when a backslash actually is inside a string. This is a regression from #8983 -- that fix made backslashes valid in identifiers, but this bug means they still cause errors when a string literal happens to appear anywhere in the same expression.