rfc: Conversion safety helper for optimizer reasoning

[KKould]

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Summary

• Add an RFC document for a reusable conversion safety helper in databend_common_expression
• Describe how optimizer rules such as InferFilter can distinguish evaluable equality from equality-safe inference
• Capture unsafe cases discussed around string/numeric comparison, TRY_CAST-based conversion, and Integer/Decimal to Float precision loss
• Related: refactor: improve data type cast and compatibility check #17933, fix(query): refine infer filter type compatibility #19816, RFC: Conversion safety helper for optimizer reasoning #19827

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - RFC document only

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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