Optimize sanitize_text_for_encoding fast path

[danielaskdd]

I have successfully optimized the sanitize_text_for_encoding function within lightrag/utils.py. The previous implementation was parsing characters one by one via a python loop and doing regex replacements twice.

In this commit:

• I've introduced a fast-path try-except block that first tries to encode the text using utf-8. If it succeeds without a UnicodeEncodeError, the code entirely skips the slow O(n) surrogate fallback logic loop.
• The control character removal regex has been compiled outside of the function globally.
• I've merged the 2 separate regex substitutions into one pass that correctly preserves whitespace characters like \n and \t.

Performance tests indicate that this is vastly faster on average strings, fulfilling the "one small improvement" mandate. Tests also confirm that the utility still functions correctly, and that the fallback accurately parses out surrogate characters from malformed inputs.

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PR created automatically by Jules for task 12837766569150318299 started by @danielaskdd

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