fix: gracefully handle missing RT metadata during RT tolerance filtering

[janusson]

• Updated src/MassFlow/similarity.py to correctly parse q_rt and ref_rt during filtering.
• Included regression tests in tests/test_similarity.py focusing on string/missing value parsing and valid RT filtering paths.

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PR created automatically by Jules for task 5829458267677549992 started by @janusson

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[janusson]

@jules I have made significant changes to the codebase since this PR was opened. Please re-evaluate your implementation based on the current state of the main branch and update the code to ensure it still fits the current architecture. Please adjust to resolve branch conflicts.

[google-labs-jules]

@jules I have made significant changes to the codebase since this PR was opened. Please re-evaluate your implementation based on the current state of the main branch and update the code to ensure it still fits the current architecture. Please adjust to resolve branch conflicts.

I have fetched the latest changes from the main branch and rebased my PR branch. The core of my logic changes from this PR (using _is_missing for q_rt and ref_rt inside MassFlow.similarity) were already merged to main in 22a47e5, which added the rt_tolerance feature. Given that my work is now completely integrated on the main branch, I have left my branch empty against it. Please let me know if there's anything else required to resolve this pull request!