Fix #6817: Trigger float_cmp_const for assert_eq! with const floats

[theoxfaber]

Fixes #6817

This PR modifies float_cmp to detect when a strict comparison of floating-point constants happens inside assertion macros (assert_eq!, assert_ne!, etc.) and correctly emits float_cmp_const instead of the general float_cmp lint. It avoids double-linting by skipping the standard check when the expression is inside an assertion macro.

changelog: Fix [float_cmp_const] not triggering on assert_eq!

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