fix(lib): correct the concat method in TokenizedStringFragments

[suojae]

The concat method was not properly updating this.fragments because Array.prototype.concat returns a new array and does not modify the original array in place. Changed the implementation to use this.fragments.push(...other.fragments); to correctly merge the fragments into this.fragments.

Description

I thought concat method in TokenizedStringFragments was intended to merge fragments from another instance into this.fragments. However, it was not functioning correctly because Array.prototype.concat returns a new array and does not modify the original array in place.

To fix this issue, I updated the concat method to use this.fragments.push(...other.fragments);. This modifies this.fragments in place and ensures that all fragments are correctly merged.

Checklist

• I have updated the PR title to match CDKTF's style guide
• I have run the linter on my code locally
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation if applicable
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works if applicable
• New and existing unit tests pass locally with my changes

[hashicorp-cla-app]

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