Harden rational edge cases and bound intermediate searches

[tusharmalpani20]

Summary

This PR improves safety and correctness for several rational-number edge cases.

Changes

• Reject division by zero with a clear PostgreSQL error.
• Correct comparisons when rational values have negative denominators.
• Prevent unsafe integer overflow in gcd() and mediant calculations.
• Reject NaN, Infinity, and out-of-range floating-point values safely.
• Add an optional maximum search depth to rational_intermediate().
• Add the 0.0.4 extension upgrade path.
• Mark rational_intermediate() functions as parallel safe.
• Add regression tests for all of the above cases.
• Update the README with the new function behavior.

Testing

• Normal PostgreSQL 18.4 build passed.
• Full regression tests passed.
• Upgrade from version 0.0.3 to 0.0.4 passed.
• Undefined-behavior and float-cast sanitizer tests passed.