fix: make Contains and collection helpers rune-safe for Unicode strings

[HarshalPatel1972]

Summary

Makes assert.Contains and underlying collection validation routines rune-safe to prevent false positives and out-of-bounds slicing bugs on multi-byte Unicode strings/emojis.

Changes

• Implemented runeSliceContains, a zero-dependency sliding window helper that evaluates []rune collections to preserve character boundary integrity.
• Updated containsElement for reflect.String variants to convert target data into []rune slices before executing structural matches.
• Added boundary validation regression tracking test scenarios inside assert/assertions_test.go.

Motivation

The previous implementation relied directly on strings.Contains, which works exclusively on raw byte tracking. If a searched token matched a specific sequence of middle-bytes within a multi-byte Unicode rune or emoji, the assertion returned invalid positive outcomes or triggered index runtime panics.

Related issues

Closes #1518

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR updates string containment checks in assert.Contains to avoid matching invalid UTF-8 byte fragments that cross Unicode rune boundaries, and adds regression tests covering Unicode-heavy scenarios.

Changes:

• Replace strings.Contains with a rune-based containment check for string inputs.
• Add new helper runeSliceContains to compare contiguous rune sub-slices.
• Add tests for Unicode behavior in Contains and ElementsMatch.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
assert/assertions.go	Switch string containment to rune-based search and add runeSliceContains helper
assert/assertions_test.go	Add Unicode-focused tests for Contains and ElementsMatch

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