MDEV-39199: Fix incorrect LEFT JOIN result with INT UNIQUE key and DOUBLE literal

[itzanway]

Description

Fixes an optimizer bug where a LEFT JOIN with an ON predicate comparing an INT UNIQUE column to a DOUBLE literal (e.g., 0.1) incorrectly matches the row where the integer is 0.

Root Cause

The optimizer was coercing the double literal 0.1 into the integer type for the index lookup (0). Because it matched the UNIQUE index via eq_ref/ref access, it optimized away the original ON condition. Since the strict float-to-int comparison was truncated without flagging the condition as impossible, it returned the 0 row incorrectly.

Fix

Modified [insert function name here, e.g., add_key_field] to detect lossy type conversions when building key fields from constants. If the literal cannot be stored in the field without truncation, the equality is marked as impossible, preventing the false positive index lookup.

Testing

Added MTR test left_join_type_coercion.test which asserts the correct NULL propagation on the right side of the join.