fix(assert): forward msgAndArgs through InEpsilonSlice

[mvanhorn]

Motivation

InEpsilonSlice calls InEpsilon once per element with the literal hint "at index %d", so the caller's msgAndArgs never reaches the failure output. The reader is left with only the relative-error line and no way to tell which assertion in a test produced it. Its sibling InDeltaSlice already forwards msgAndArgs..., so the two slice helpers behave differently for identical caller code.

Summary

Forward the caller's message to each per-element InEpsilon call while keeping the index hint, so a failure names both the mismatching element and the caller's context.

Changes

• assert/assertions.go: build the per-element message as at index N plus the caller's rendered msgAndArgs (via the existing messageFromMsgAndArgs) when any were supplied, and pass that to InEpsilon.
• assert/assertions_test.go: add TestInEpsilonSliceMessage, asserting the complete rendered failure output for a call with a formatted message, for a call without one, and that a passing comparison still writes nothing.

The signature is unchanged, so the generated forwarders in assert/assertion_format.go, assert/assertion_forward.go, and require need no regeneration. Callers that pass no message keep exactly their current output.

Related issues

This supersedes #1899, which made the same forwarding change and was closed by its author without answering the one review question it received: whether the added test could assert the full error rather than a fragment. The test here does assert the full output, matching the convention the existing message tests in this file already use.

Closes #763